Your letter sounds a little passive as well. If it leaked so much that you had to supplement with your own DME to make up for the loss of gravity, there is a problem. I would have simply taken pictures and told them I want to return the kit. A "backup bag" doesn't mean s***. You don't need to reassure the store that you'll be shopping there again either. Give a store praise and promise when they do good things, not reassurance when they do something bad.