Reply To: Paypal button to pay after deposit

Did you used the email tag like [paypal 20] to set the deposits amount?
The normal payment deposit setting is just for the form.

If you mean the mail after payment it can be that the payment IPN isn’t working in sandbox. That can have various reasons, for example SSL must be activated in the sandbox. Sometimes the IPN log in the paypal settings can be helpful to find the problem.