Not from Mexico, so it might be different for them, but one way how this works in some places is that the tax office gets all the electronic receipts from all the cash register/POS systems anyway, since that's how they monitor sales/vat/excise/etc taxes paid by the merchant and reduce the risks of "under the counter" sales where fake receipts are issued but taxes not paid on the sale.

So if the doctor's office punches in my legal ID number with the receipt (or if the ID gets added automatically from the billing system that a large hospital would have), then when the office sends all their receipt files to the tax office, the tax office will automatically add that electronic receipt to my tax deduction.