diff --git a/backend/internal/service/payment_order_result_test.go b/backend/internal/service/payment_order_result_test.go
index bfe275481c..3a96df54b8 100644
--- a/backend/internal/service/payment_order_result_test.go
+++ b/backend/internal/service/payment_order_result_test.go
@@ -138,6 +138,34 @@ func TestCalculateCreateOrderPayAmountRejectsFractionalZeroDecimal(t *testing.T)
 	}
 }
 
+func TestComputeValidityDaysSupportsSingularAndPluralUnits(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		days int
+		unit string
+		want int
+	}{
+		{name: "days", days: 1, unit: "days", want: 1},
+		{name: "week", days: 1, unit: "week", want: 7},
+		{name: "weeks", days: 2, unit: "weeks", want: 14},
+		{name: "month", days: 1, unit: "month", want: 30},
+		{name: "months", days: 1, unit: "months", want: 30},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := psComputeValidityDays(tt.days, tt.unit); got != tt.want {
+				t.Fatalf("psComputeValidityDays(%d, %q) = %d, want %d", tt.days, tt.unit, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestBuildPaymentSubjectAppliesAffixToSubscriptionPlanProductName(t *testing.T) {
 	t.Parallel()
 
diff --git a/backend/internal/service/payment_service.go b/backend/internal/service/payment_service.go
index 2759aba145..c1deb667ef 100644
--- a/backend/internal/service/payment_service.go
+++ b/backend/internal/service/payment_service.go
@@ -337,15 +337,17 @@ func psSliceContains(sl []string, s string) bool {
 
 // Subscription validity period unit constants.
 const (
-	validityUnitWeek  = "week"
-	validityUnitMonth = "month"
+	validityUnitWeek   = "week"
+	validityUnitWeeks  = "weeks"
+	validityUnitMonth  = "month"
+	validityUnitMonths = "months"
 )
 
 func psComputeValidityDays(days int, unit string) int {
 	switch unit {
-	case validityUnitWeek:
+	case validityUnitWeek, validityUnitWeeks:
 		return days * 7
-	case validityUnitMonth:
+	case validityUnitMonth, validityUnitMonths:
 		return days * 30
 	default:
 		return days