Hmm. I didn't think of reimbursement. It's a good thing I am not actually in charge. But no, I was not planning on different transaction types. My reasoning is that if you treat business-customer, business-business, differently then it adds extra friction for interpersonal commerce. Under our current tax system if I want to hire my neighbor to mow my lawn now and then, I need to register myself as an employer, get a tax id, file a w-4, then submit yearly w-2s. Or he could register as a contractor and do it himself. In either case it is a massive pain in the neck, so for odd jobs we either go under the table or just don't hire each other at all.
If your law requires people become (minor) criminals just to associate freely, then something is wrong. The case of reimbursement is an issue or not depending on the transactional tax rate. We seem to be ok already with 3% going to credit card processors.
You probably can't run a government on 3% But I think the size of the economy would be much larger with greater transactional velocity.
In your flat tax would everyone pay the same amount or is it a rate on income?