I think Advance will sell you as much as you want. Shell will only give you the rebate on 1 though. You could buy 1, send in the rebate. Go in the next day and buy one, send in another rebate for a total of 2.
"Limit 2 rebates per person, household or address (e-mail and physical)"
My Advance Auto store has it for $22 but it's $13.99 on the website.
I'm printing the page and plan to see what I can talk them in to.
What's the worst they can say, no?
$22 is still OK for something I'll buy anyways.