diff --git a/src/modules/bills/bills.service.ts b/src/modules/bills/bills.service.ts
index b41f56e..dfe2a9b 100644
--- a/src/modules/bills/bills.service.ts
+++ b/src/modules/bills/bills.service.ts
@@ -49,7 +49,10 @@ export class BillsService {
       const companyMap = new Map<string, Company>(companies.map((c) => [c.id, c]));
 
       for (const value of values) {
-        const subtotal = new Decimal(waterRate).mul(value.waterUsage);
+        const waterCost = new Decimal(waterRate).mul(value.waterUsage);
+        const sewageCost = waterCost.mul(waterCost);
+        const subtotal = waterCost.add(sewageCost);
+
         const tax = subtotal.mul(0.1);
         const totalPrice = subtotal.add(tax);
         const company = companyMap.get(value.companyId);
diff --git a/src/modules/payments/constants/index.ts b/src/modules/payments/constants/index.ts
index 3c109c8..e1d8fea 100755
--- a/src/modules/payments/constants/index.ts
+++ b/src/modules/payments/constants/index.ts
@@ -1,6 +1,9 @@
 export const ZARINPAL_CONFIG = "ZARINPAL_CONFIG";
 //===============================================
 
+/** Payment currency: IRR = Iranian Rial, IRT = Iran Toman (1 Toman = 10 Rials) */
+export const PAYMENT_CURRENCY = "IRT" as const;
+
 export const PAYMENT = Object.freeze({
   QUEUE_NAME: "payment",
   START: "payment_start",
diff --git a/src/modules/payments/gateways/zarinpal.gateway.ts b/src/modules/payments/gateways/zarinpal.gateway.ts
index 2f75823..1ecc30d 100755
--- a/src/modules/payments/gateways/zarinpal.gateway.ts
+++ b/src/modules/payments/gateways/zarinpal.gateway.ts
@@ -5,7 +5,7 @@ import { catchError, firstValueFrom } from "rxjs";
 
 import { PaymentMessage } from "../../../common/enums/message.enum";
 import { IZarinpalConfig } from "../../../configs/zarinpal.config";
-import { ZARINPAL_CONFIG } from "../constants";
+import { PAYMENT_CURRENCY, ZARINPAL_CONFIG } from "../constants";
 import { GatewayEnum } from "../enums/gateway.enum";
 import {
   IPaymentGateway,
@@ -36,7 +36,7 @@ export class ZarinpalGateway implements IPaymentGateway {
         amount: processParams.amount,
         callback_url: `${this.config.callBackUrl}/${GatewayEnum.ZARINPAL}`,
         description: processParams.description,
-        currency: "IRT",
+        currency: PAYMENT_CURRENCY, // IRT = Iranian Rial
         metadata: { email: processParams.email, mobile: processParams.mobile },
       };
 
diff --git a/src/modules/payments/services/payments.service.ts b/src/modules/payments/services/payments.service.ts
index 58431f2..424900c 100644
--- a/src/modules/payments/services/payments.service.ts
+++ b/src/modules/payments/services/payments.service.ts
@@ -57,6 +57,7 @@ export class PaymentsService {
 
       if (invoice.company.user.id !== user.id) throw new BadRequestException(InvoiceMessage.NOT_FOUND_BY_ID_OR_NOT_BELONG_TO_USER);
 
+      // Amount is in IRT (Iranian Toman)
       const gatewayData = await this.processPayment(paymentGateway.name, {
         amount: new Decimal(invoice.totalPrice).toNumber(),
         description: PaymentMessage.CHECKOUT_INVOICE_MESSAGE.replace("[invoiceNumber]", invoice.numericId.toString()),