Can't find the right forum.

I'm not a big fan of the Visa gift card. I got a couple of the $100 cards and each time i go to a restaurant and the bill is over $80 my card get's rejected. I guess they tack on a 20% tip and it would be rejected if the bill is more than 80+anticipated 20% ($20) tip. How can I avoid this so I can use the full value of my card? IS it impossible at a restaurant? Kind of defeats the purporse of withholding 20% but won't let you use it! How about letting me use the full value and I tip separately.