My state has a WIC card. Super, super easy to do! We scan all items like normal, guest runs their WIC card, we get prompted to print a receipt that shows the guest their beginning WIC balance, hand it to the guest (they never read it), they enter their pin #, out comes their ending balance receipt, and then the guest pays the remaining balance. Remaining balance is for non-WICable food items, non-food, whatever.
If they have SNAP too, that card gets swiped after WIC.
It's all very easy and much easier to process than paper vouchers.