Add to your policies everywhere and specially on purchase pages, that a user can get refund within 3 hours.
(you may increase timeframe for sure)
Let user accept the same.. Add this as a checkbox to accept before user continues to purchase..
Let them purchase and don't refund back if users do not respond within specified time period.
(I would say 24 hours as best or 48 hours depending on nature of products which unsure)
your competition is playing with you
Talk to your crediting bank, let them know/understand about the policy you have on your site.
These issues are common when you are a fish and deal with CROCODILES in RIVER..
(E-COMMERCE WE SAY)
It will work for sure, It worked for many like a charm..
Give it a try.