Some Sims sites aren't precisely pay sites but they do offer a gift with donation, like a tanning bed.
The problem is that Paysites Must Be Destroyed takes all remotely "pay" content and redistributes it for free. Ignoring the questionable ethics behind their methods, pay and gift sites cannot keep redistribution from happening with their game items.
I had an idea, and it could be a good or bad idea, but how about offering something that isn't game content for donations? Like, $5 gets you a book of stickers in the mail or $10 gets you a handful of points for the official Sims 3 store. Yes, it would cut into donation profits but it would also satisfy the urge to give something back to donors while making the gift something that can't be redistributed.
Heck, Mod the Sims 2 just gives you a star by your name! Talk about impossible to steal.
In a perfect world, no one would have to worry about piracy and all content (and games) would be free, but this world isn't perfect. I'm just suggesting that if you want to make your donation gifts exclusive, this is one option.
Design something on CafePress that is unique to your site and style. The best creators are usually terrific artists, so I know this is possible. Stickers, buttons, magnets -- these things can be bought in bulk by the creator for cheap.
Take requests from donors who hit a certain donation mark. Everyone will benefit by the new content but your donors get exactly what they want.
Spend a little of your bigger donations handing out points for the official Sims 3 store. This gives your visitors content they usually won't have access to (unless PMBD takes official content, too) and you get the satisfaction of their having more content at the end of the day.