Finetuning Zencart store

I've had someone upgrade a zencart store for me. They've upgraded from 1.3.7 to 1.3.8 & added a number of pluggins. Now I need to someone to help fine tune the new store as the current designer is having problems. The items that need 'finishing': 1. On testing the store, customers cannot checkout - just get a blank white screen. 2. Need to complete transfer of customer database from the the old to the new store. 3. the new store has a Superorders pluggin which allows labels to be printed for selected orders. I need the labels php file to be adapted so it includes my return address an postage image.