<form target="_parent" ... />
Using Kevin Vella's idea, I tried using the above on the form element made by PayPal's button generator. Worked for me so that Paypal does not open in a new browser window/tab.
Update
Here's an example:
Generating a button as of today (01-19-2021), PayPal automatically includes target="_top"
on the form
element, but if that doesn't work for your context, try a different target
value. I suggest _parent
-- at least that worked when I was using this PayPal button.
See Form Target Values for more info.
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_parent">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="[email protected]">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="button_subtype" value="services">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHostedGuest">
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_buynowCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>