The question is very muddled, as it seems to be mixing unrelated topics - the technologies for developing a coupon system (whatever you mean by that, as it isn't entirely clear if you are talking about transferrable discounts or something else; also, note the spelling of 'coupon'), the legal requirements for a coupon system, and the tax code in your local jurisdiction, just to name a few.
As rproffit stated, the best person to discuss the legalities of such a system would be legal counsel (presumably one specializing in contract law as it applies to transferrable discount coupons), not a technical forum. Similarly, the issues of tax codes would be better referred to a CPA, or more likely, a lawyer specializing in tax law (which is a very different topic from contract law vis a vis discounting, which means you'd need to talk to two lawyers at minimum).
None of which relates to the technical side of how to compute the tax applied to these discounts, though you won't be able to write your code for that without speaking to those experts first.
The statements and questions regarding the technology used are, to be blunt, baffling to me. They are so general and uninformative that there simply isn't any substance to what you've said which I can comment on. Could you please explain your intended design in greater detail (without revealing any proprietary information, of course)?