This is exactly what happened to me. I reached out to support, they said the promo was applied in the first conversation directly after the order and then got told tonight that "they can't find an account with your email address". Despite the fact I was literally on my account page looking at the email and everything. I'm going to go out on a limb and say that I'm not getting the pizza emote.
I asked for a transcript of my chats, so I may post them later if this emote doesn't show up. Just so everyone can see they lied to my face.
Also... it took them 5 hours to deliver a pizza to me. The pizza place I ordered from felt so guilty, they upgraded my pizza to a large and added a coke and a cookie for free. GrubHub is absolute bollocks and you bet I'm never using them again.