Might help you to get a scale and just send them out at a competitive rate. Hell, you wouldn't even need the scale if you were just selling covers. They all weigh the same. Just do some multiplication.
Should cost about $4-5 anywhere in the US in a bubble mailer first class. One cover is certainly under 16oz.
Anything over 16oz would go Priority and ship for $8-11 anywhere in the US. Would be the same on up to about 5lb.
If you're gonna be asking a hefty price for the cover you may want to figure out a system so you don't hit people over the head on shipping.
@Woodslasher has an incredibly valid point.
$22 would scare a lot of folks off.
Think about getting the shipping right, it'll help you get a return on your investment.