Sadly, I haven't gotten a DBC dollar-denominated voucher in a long time; the terms & conditions used to make clear that you could freely combine vouchers from the same booklet, but could not combine vouchers from different DBC booklets.

The fundamental differentiation is that some vouchers are considered "Type A", or discounts; others are considered "Type B", or a form of payment. Your service check is probably a "Type A" voucher; your DBC vouchers are "Type B" vouchers.

Some vouchers will say that they're not combinable with other vouchers or certificates, but even then, knowledgeable agents usually will allow you to combine a "Type A" discount voucher with a "Type B" payment voucher. Your mileage will definitely vary.