Jacques makes a good point that you should be careful with customers seeing started but not completed transactions. Any confusion on the customer's part is bad

Jacques makes a good point that you should be careful with customers seeing started but not completed transactions. Any confusion on the customer's part is bad news. If you're going to go the route of authorization only then capture, then I'd suggest that you invalidate authorizations yourself after a set period of time (maybe a few minutes?) rather than let the gateway invalidate it based on their criteria. This doesn't, however, solve your problems with the number of requests sent to the gateway. I don't know much about MercuryPay (and their website is scarce on in-depth developer info), but maybe you could work with them on posting payment information and holding it until the transaction is confirmed?