1) out of the goodness of your heart
2) to promote the local economy
3) to spread the wealth and overcome income inequality
4) because you forgot to give them the Deal or Coupon code, or you did give it to them but they failed to apply it to the invoice
5) brain freeze so you forgot you had already paid so paid twice
6) its Halloween - a zombie ate your brain
Normally, with a coupon or deal, you would pay AL the price for the Deal or coupon, present it or the code to the vendor to pay for the work - or at least for the value of the Deal or coupon, then pay the vendor directly for anything agreed to in the bid/estimate/contract above that amount.
If you are still confused, I would double check the invoice against the Deal or coupon terms, then talk to the vendor - because once work has been done AL cannot help much unless you have a complaint that the vendor is not honoring his Deals/Coupons, in which case contact member care at AL - memberservices@angieslist.com 1-866-783-2980. Call center hours are Monday through Friday 8:00 am-9:00 pm Eastern Time or Saturday 8:00 am-5:00 pm ET.