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Actually, customers can buy multiple product from your form, but you don't see it instantly. If your Paypal tool has more than one product, user can choose among them and when they hit the "Submit" button, selected ones are ready to be sold on Paypal's webpage. So, I've created a form and added this option. You can check it from the below:
To add an total amount into your paypal payment tool,
1) Open the Paypal wizard.
2) Check the "Show total on the form" checkbox.
As it is seen on the demo, when you increase the quantity, the total value is going up.
If you encounter any error, please let us know.
Have a nice day.