Web Forms :: Append CustomPrice To The Current Page's Query String Before The Response.Redirect Takes Place

I have a page that is used to add info from a gridview into a user's profile. I have explained my issues after all of the code. Here is the applicable code(s) involved:

Protected Sub AddToCart(ByVal sender As Object, ByVal e As CommandEventArgs)
For Each gvr In GridViewItems.Rows
Dim txtGiftAmt As TextBox = (CType(gvr.FindControl("txtGiftAmt"), TextBox))
If txtGiftAmt.Visible = True Then
Dim customPrice As String = txtGiftAmt.Text.ToString
'HERE is where I would like to set the new price variable value somehow
End If
Next
Dim scart As New ShoppingCart
scart.AddCartItem(e.CommandArgument.ToString())
Response.Redirect("~/ShoppingCart.aspx")
End Sub

This sub is called by the following button when clicked:

[Code]....

Here is the sub from the ShoppongCart Class named AddCartItem:

[Code]....

Here is the problem that I am having: I have one specific item that allows user's to define their own price (for this item only). By putting this price in a session variable seems to be the easiest way to handle this (at least for me), but the session variable's value always "lags behind". By this I mean that the first time the items is entered, it uses the default value of cartItem.ItemPrice. The second time the same item is added (with the intention of a different price), this new item's cartItem.ItemPrice is set
as the first session("customPrice").

Because the user is selecting his/her own price for this item, I am not worried about lack of security involved with using query string to pass this price, but I can't get the string passed to the ShoppingCart class correctly (shows up null). Is there a way to append this customPrice to the current page's query string before the Response.Redirect takes place? If I understand this correctly, the Response portion "posts back" to the current page before the Redirect sends us on our merry way, right?