Hi!
To leave the 'amount' empty for the buyer to enter during checkout, simply leave the 'Amount' field during the creation of the button.
If you have already created the button and it's a 'hosted button' type button ('Host button with PayPal is ticked in Step 2), you'll need to re-create the button and leave the amount field empty upon creation.
If you don't have a hosted button ('Host button with PayPal is unticked in Step 2 of the button creation tool), simply remove the appropriate line from your button's HTML code.
E.g. remove <input type="hidden" name="amount" value="x.x">
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For technical assistance with PayPal merchant product offerings, please file a ticket at https://www.paypal.com/mts/
