Hi Paul.
The payment gateways don’t give that option…
What you could do, although not as part of a subscription trial, rather as a trial your site offers and then would need to get the person to enter the payment details if they want more access, would be using an approach similar to content dripping. WP Admin > s2Member > API / Scripting > Content Dripping
See also: https://s2member.com/kb-article/s2drip-shortcode/
Or maybe let people register for free with the higher access level, and then have every page check if the user registered over 2 hours ago but didn’t pay, and drop his level to 0 so he has to pay to get it back. https://s2member.com/kb-article/rolescapabilities-via-php/
I hope that helps.