The Disney Gift Cards they sell at H-E-B are of the $15-$500 variety. The Yellow coupon says $5 off a $50 purchase of 6 random cards, one of which is the Disney card.
So each transaction, buy one card, ask for it to loaded at $50, and give the cashier the coupon, dropping the price to $45.
I would take 3 Disney Gift Cards and 3 coupons to the register, and tell them you want to do these in 3 different transactions. That way, you are respectful to the other people checking out. Then just get back in line with 3 more, etc. until you have as many as you want. That way has worked for me several times in the past.
Good luck!