We used to process some bulk-loaded films for customers. However, we weren't undertaking mailorders, they would drop off and then collect. As the cannisters were manually removed from the machines, putting them backinto their order envelopes wasn't a problem.
If you have concerns regarding the "bulk loading" ones, maybe try those that are discarded by the labs. That way it won't matter if they are not returned and are basically free to you. We had a number of people who came in every month or so and asked for empty cans. I used to fill a RA4 paper bag for each of them and once it was full, sealed and named it for them to collect. When you are processing 300 or so rolls a week, it fills pretty quickly! Obviously these days d***** has eaten away at those numbers, but you could still get enough cans to last quite a while.
Just make sure you tape the film very securely to the remaining film in the can; more than once I had to rescue film simply because they had used ordinary scotch tape to attach. Needless to say these didn't survive very well - minilabs (unlike many pro-labs) aren't kept in separate light-tight rooms. And the only way to get them out is to open the lid!
I used to sell rolls of the splicing tape to a few of my regulars - this was good stuff that attached securely and was chemical resistant. Depending upon the relationship you have with your local lab, they might be able to help you out there too.