can't POST item price in paypal sandbox

Answer 1

The reason you can't override the amount dynamically, is because you have a so-called 'PayPal hosted button'.
With a hosted button, the amount is stored on PayPal's side and can't be overwritten with the 'amount' variable. You'll either want to use a non-hosted button, or use the BMUpdateButton API call to dynamically update the button's amount.

An example request for BMUpdateButton would look as follows:

USER=Your API username
PWD=Your API password
SIGNATURE=Your API signature
VERSION=82.0
HOSTEDUBTTONID=The value of <input type="hidden" name="hosted_button_id" value="">
BUTTONTYPE=The type of button. E.g. BUYNOW
BUTTONCODE=The type of code you want to get back. E.g. HOSTED
L_BUTTONVAR0=amount=The new amount with a period as separator
L_BUTTONVAR1=item_name=Optional: a new item name if you wish

Similary, you could also use the BMCreateButton API to create a new button, or use the BMButtonSearch API to search through a list of all your stored hosted buttons (to find the hosted_button_id of your button automatically, for example)

The reason to use a hosted button is because it's more secure. A non-hosted, unencrypted button would basically leave the amounts open to manipulation. Fraudulent transactions waiting to happen.