fix: end_shipper_id param

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Removes the deprecated, unusable `ReferralCustomer.AddCreditCardToUser` function
   - Stripe has disabled the ability to pass plain credit card details over the wire and now requires using [Stripe.js/Elements/Checkout](https://support.stripe.com/questions/card-tokenization-restrictions-using-publishable-keys). Follow the [Decentralized (EasyPost-Manage Billing) Guide](https://docs.easypost.com/guides/get-started-with-forge/easypost-managed-billing-guide#referralcustomer-billing-management) for more details on the new flow to use.
   - Makes `ReferralCustomer.RetrieveEasypostStripeApiKey` public to help facilitate adding credit cards using Stripe.js
+- Correctly passes `end_shipper_id` param instead of `end_shipper` in shipment buy calls (closes [669](https://github.com/EasyPost/easypost-csharp/issues/669))
 
 ## v7.10.0 (2026-07-24)
 

EasyPost/Parameters/Shipment/Buy.cs

@@ -27,7 +27,7 @@ public class Buy : BaseParameters<Models.API.Shipment>
         /// <summary>
         ///     The ID of the <see cref="Models.API.EndShipper"/> to buy the <see cref="Models.API.Shipment"/> with. Optional.
         /// </summary>
-        [TopLevelRequestParameter(Necessity.Optional, "end_shipper")]
+        [TopLevelRequestParameter(Necessity.Optional, "end_shipper_id")]
         public string? EndShipperId { get; set; }
 
         #endregion

EasyPost/Services/ShipmentService.cs

@@ -196,7 +196,7 @@ public async Task<Shipment> Buy(string id, string? rateId, string? insuranceValu
 
             if (endShipperId != null)
             {
-                parameters.Add("end_shipper", endShipperId);
+                parameters.Add("end_shipper_id", endShipperId);
             }
 
             return await RequestAsync<Shipment>(Method.Post, $"shipments/{id}/buy", cancellationToken, parameters);