When Posting a Cart with Voluntary Contributions inside of it, values get reset to default after posting

I am attempting to post a cart with a Voluntary Contribution inside of the cart. The user enters a value and that value is added to the payload that I am posting. It is successful on post, but as soon as I go to the cart to view the items, the value gets overwritten to the default value for that voluntary contribution

"$type": "Asi.Soa.Commerce.DataContracts.OrderLineData, Asi.Contracts",
            "OrderLineId": "",
            "ChildOrderLines": {
                "$type": "Asi.Soa.Commerce.DataContracts.OrderLineDataCollection, Asi.Contracts",
                "$values": [
                    {
                        "$type": "Asi.Soa.Commerce.DataContracts.OrderLineData, Asi.Contracts",
                        "OrderLineId": "",
                        ...(payloadType === "VOL" && {
                            "ExtendedAmount": {
                                "$type": "System.Nullable`1[[Asi.Soa.Core.DataContracts.MonetaryAmountData, Asi.Contracts]], mscorlib",
                                "Amount": donation,
                                "Currency": {
                                    "$type": "Asi.Soa.Core.DataContracts.CurrencyData, Asi.Contracts",
                                    "CurrencyCode": "USD",
                                    "DecimalPositions": 2
                                },
                                "IsAmountDefined": true
                            }
                        }),

The donation value in the "Amount" field is populated by the user entering a value. There are default values stored in the system for each voluntary contribution: Voluntary Contribution to the Foundation: 0.00, PAC Contribution: 20.00, Scholarship Fund Contribution: 30.00. When I post the cart, the cart is posted with the value that the user entered, but as soon as I go to view the cart, the value has been overwritten.

Where/How are the default values stored and is there a way to bypass the default values