It is not possible at this time for a custom button and a FASO generated button to show up in the same shopping cart.
Essentially, the custom Paypal buttons are directing the buyer to a cart that is managed by Paypal whereas the FASO-based Paypal buttons are directing the buyer to a FASO-managed shopping cart.
Therefore, a transaction for purchasing items that use a custom Paypal button along with items that use a FASO-based Paypal button will fail since they are 2 separate shopping carts.
At the moment, the only workaround would be to have all items use a custom button so that all items open in the Paypal managed shopping cart.
Although there are plans to eventually address this at this point in time, the FASO development team does not have any concrete timeframe to begin work on integrating the two shopping cart systems.
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