Distribution/Features/Scheme/ISchemeRepository.cs

@@ -11,6 +11,7 @@ public interface ISchemeRepository
     Task<List<Models.SchemeRedemption>> GetRedemptionsAsync(Guid schemeId);
     Task AddRedemptionAsync(Models.SchemeRedemption entity);
     Task<bool> HasRedemptionAsync(Guid schemeId, Guid orderId);
+    Task<bool> HasLockedInvoiceAsync(Guid shopPOId);
 
     Task<Models.SchemeBundleComponent?> GetBundleComponentAsync(Guid schemeId, Guid itemId);
     Task AddBundleComponentAsync(Models.SchemeBundleComponent entity);

Distribution/Features/Scheme/SchemeRepository.cs

@@ -52,6 +52,10 @@ public class SchemeRepository(AppDBContext context) : ISchemeRepository
     public async Task<bool> HasRedemptionAsync(Guid schemeId, Guid shopPOId) =>
         await context.SchemeRedemptions.AnyAsync(r => r.FK_SchemeId == schemeId && r.FK_ShopPOId == shopPOId);
 
+    // An invoice past Draft (printed or finalized) has a fixed amount, so it can no longer take a discount.
+    public async Task<bool> HasLockedInvoiceAsync(Guid shopPOId) =>
+        await context.Invoices.AnyAsync(i => i.FK_ShopPOId == shopPOId && i.IsActive && i.Status != Models.InvoiceStatus.Draft);
+
     public async Task<Models.SchemeBundleComponent?> GetBundleComponentAsync(Guid schemeId, Guid itemId) =>
         await context.SchemeBundleComponents
             .FirstOrDefaultAsync(c => c.FK_SchemeId == schemeId && c.FK_ItemId == itemId);

Distribution/Features/Scheme/SchemeService.cs

@@ -383,6 +383,9 @@ public class SchemeService(
         if (await repo.HasRedemptionAsync(schemeId, shopPOId))
             throw new InvalidOperationException("This scheme has already been redeemed against this ShopPO.");
 
+        if (await repo.HasLockedInvoiceAsync(shopPOId))
+            throw new InvalidOperationException("This order's invoice is already printed or finalized, so a scheme can no longer be applied. Apply schemes before the invoice is printed.");
+
         var items = await shopPORepo.GetActiveItemsByShopPOIdAsync(shopPOId);
         var eligibility = EvaluateRule(scheme, items);
         if (!eligibility.Eligible)