Yeah if the coupon is not available to scan or won't scan the clerk must type in the coupon code manually. If you have a smartphone, you can log into the Mobile version of the Powerup site and the clerk can get it from that or you have to write the coupon code down as Denbo said.
Gamestop stores don't have internet access. Almost all have a broadband connection but it's locked down. It's to connect the registers to the servers in TX for all the transactions and also to get the DLC codes from MS/Sony, an intranet site for info on promotions, and for the management team to get emails.