Upgrading Pre-paid Orders - Q For Tim Or Anyone Please!

Hi Everyone.

For those of you selling prepaid packages upfront whereby the client comes back to choose their images ( Ie:  Client prepays a cheap 4-pic pack, comes back when the gallery is ready and is AMAZED by the 10 you took and now wants to abandon prepay for a better pack .. )

A $40 prepay sale is now a $120 sale

Is there an easy way to upsell/upgrade for the client?

I know the client can fulfil prepay and then come back for a round 2 purchase, but i'd rather they don't check out and make the one sale, I really really really want them to spend more money so i'd like to have some irresistable upgrades and the difference is paid at checkout?

thankyou to all that read and reply :)