If you plan on supporting more than one store, you need create a solution that works for more than one store, so either ditch vouchers altogether and leave it all to 3rd party or, preferably, make them global. Your solution puts all kinds of constraints on you most successful customers (the ones that want to grow), e.g., vouchers, global customers that you can't edit, a store manager/POS that can't see stock outside of the store...
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