diff --git a/contracts/cntr/Cargo.toml b/contracts/cntr/Cargo.toml
index 40fdfbc..1bb6c17 100644
--- a/contracts/cntr/Cargo.toml
+++ b/contracts/cntr/Cargo.toml
@@ -2,3 +2,7 @@
 name = "cntr"
 version = "0.1.0"
 edition = "2021"
+publish = false
+
+[lib]
+doctest = false
diff --git a/contracts/cntr/src/grace_period.rs b/contracts/cntr/src/grace_period.rs
new file mode 100644
index 0000000..888cd05
--- /dev/null
+++ b/contracts/cntr/src/grace_period.rs
@@ -0,0 +1,84 @@
+/// Default grace period: 3 days in seconds.
+pub const DEFAULT_GRACE_SECONDS: u64 = 259_200;
+
+#[derive(Debug, PartialEq)]
+pub enum SubscriptionStatus {
+    Active,
+    InGracePeriod,
+    Expired,
+}
+
+/// Returns the subscription status based on timestamps.
+///
+/// - `Active`        when `current_ts < expiry_ts`
+/// - `InGracePeriod` when `expiry_ts <= current_ts < expiry_ts + grace_seconds`
+/// - `Expired`       when `current_ts >= expiry_ts + grace_seconds`
+pub fn get_subscription_status(
+    expiry_ts: u64,
+    current_ts: u64,
+    grace_seconds: u64,
+) -> SubscriptionStatus {
+    if current_ts < expiry_ts {
+        SubscriptionStatus::Active
+    } else if current_ts < expiry_ts.saturating_add(grace_seconds) {
+        SubscriptionStatus::InGracePeriod
+    } else {
+        SubscriptionStatus::Expired
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EXPIRY: u64 = 1_000_000;
+    const GRACE: u64 = DEFAULT_GRACE_SECONDS;
+
+    #[test]
+    fn active_well_before_expiry() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY - 1000, GRACE),
+            SubscriptionStatus::Active
+        );
+    }
+
+    #[test]
+    fn active_one_second_before_expiry() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY - 1, GRACE),
+            SubscriptionStatus::Active
+        );
+    }
+
+    #[test]
+    fn in_grace_period_at_exact_expiry() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY, GRACE),
+            SubscriptionStatus::InGracePeriod
+        );
+    }
+
+    #[test]
+    fn in_grace_period_one_second_before_grace_ends() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY + GRACE - 1, GRACE),
+            SubscriptionStatus::InGracePeriod
+        );
+    }
+
+    #[test]
+    fn expired_at_exact_grace_boundary() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY + GRACE, GRACE),
+            SubscriptionStatus::Expired
+        );
+    }
+
+    #[test]
+    fn expired_well_past_grace() {
+        assert_eq!(
+            get_subscription_status(EXPIRY, EXPIRY + GRACE + 10_000, GRACE),
+            SubscriptionStatus::Expired
+        );
+    }
+}
diff --git a/contracts/cntr/src/lib.rs b/contracts/cntr/src/lib.rs
index f0def92..d8e5eb4 100644
--- a/contracts/cntr/src/lib.rs
+++ b/contracts/cntr/src/lib.rs
@@ -1,3 +1,5 @@
+pub mod grace_period;
+pub mod payment_validator;
 pub mod credit_topup;
 pub mod multi_booking;
 pub mod payment_validator;
diff --git a/contracts/cntr/src/payment_validator.rs b/contracts/cntr/src/payment_validator.rs
index 0ca7a2e..0540179 100644
--- a/contracts/cntr/src/payment_validator.rs
+++ b/contracts/cntr/src/payment_validator.rs
@@ -1,3 +1,32 @@
+/// Default payment tolerance in stroops (1 XLM = 10_000_000 stroops).
+pub const DEFAULT_TOLERANCE: i128 = 100;
+
+#[derive(Debug, PartialEq)]
+pub enum PaymentError {
+    NegativePayment,
+    ZeroPayment,
+    Underpayment,
+}
+
+/// Validates that `paid_amount` satisfies `expected_amount` within `tolerance`.
+///
+/// Accepts if `paid_amount >= expected_amount - tolerance`.
+/// Overpayment is always accepted.
+pub fn validate_payment(
+    paid_amount: i128,
+    expected_amount: i128,
+    tolerance: i128,
+) -> Result<(), PaymentError> {
+    if paid_amount < 0 {
+        return Err(PaymentError::NegativePayment);
+    }
+    if paid_amount == 0 && expected_amount > 0 {
+        return Err(PaymentError::ZeroPayment);
+    }
+    if paid_amount < expected_amount - tolerance {
+        return Err(PaymentError::Underpayment);
+    }
+    Ok(())
 /// Validates that a received payment amount is within tolerance of the expected amount.
 ///
 /// Returns `Ok(())` when `received_stroops >= expected_stroops - tolerance_stroops`.
diff --git a/contracts/cntr/tests/payment_verification_tests.rs b/contracts/cntr/tests/payment_verification_tests.rs
new file mode 100644
index 0000000..44a6d3d
--- /dev/null
+++ b/contracts/cntr/tests/payment_verification_tests.rs
@@ -0,0 +1,75 @@
+use cntr::payment_validator::{validate_payment, PaymentError, DEFAULT_TOLERANCE};
+
+const EXPECTED: i128 = 10_000_000; // 1 XLM in stroops
+const TOL: i128 = DEFAULT_TOLERANCE;
+
+#[test]
+fn exact_amount_passes() {
+    assert!(validate_payment(EXPECTED, EXPECTED, TOL).is_ok());
+}
+
+#[test]
+fn overpayment_passes() {
+    assert!(validate_payment(EXPECTED + 1_000_000, EXPECTED, TOL).is_ok());
+}
+
+#[test]
+fn within_tolerance_passes() {
+    assert!(validate_payment(EXPECTED - TOL, EXPECTED, TOL).is_ok());
+}
+
+#[test]
+fn one_stroop_below_tolerance_fails() {
+    assert_eq!(
+        validate_payment(EXPECTED - TOL - 1, EXPECTED, TOL),
+        Err(PaymentError::Underpayment)
+    );
+}
+
+#[test]
+fn underpayment_well_below_fails() {
+    assert_eq!(
+        validate_payment(EXPECTED / 2, EXPECTED, TOL),
+        Err(PaymentError::Underpayment)
+    );
+}
+
+#[test]
+fn zero_payment_fails() {
+    assert_eq!(
+        validate_payment(0, EXPECTED, TOL),
+        Err(PaymentError::ZeroPayment)
+    );
+}
+
+#[test]
+fn negative_payment_fails() {
+    assert_eq!(
+        validate_payment(-1, EXPECTED, TOL),
+        Err(PaymentError::NegativePayment)
+    );
+}
+
+#[test]
+fn zero_expected_amount_with_zero_paid_passes() {
+    assert!(validate_payment(0, 0, TOL).is_ok());
+}
+
+#[test]
+fn tolerance_of_zero_exact_passes() {
+    assert!(validate_payment(EXPECTED, EXPECTED, 0).is_ok());
+}
+
+#[test]
+fn tolerance_of_zero_one_stroop_under_fails() {
+    assert_eq!(
+        validate_payment(EXPECTED - 1, EXPECTED, 0),
+        Err(PaymentError::Underpayment)
+    );
+}
+
+#[test]
+fn large_amount_near_i128_max_passes() {
+    let large = i128::MAX / 2;
+    assert!(validate_payment(large, large, TOL).is_ok());
+}