these are the sizes i keep on hand for my students.
16x20 matts with windows both landscape and portrait for 8x10 prints; which are really a bit smaller with the 1/4 inch wide border from the easel. and 6x9 windows , both landscape and portrait. All are bottom weighted.

Rather than using the standard window for 8x10 and 6x9, i have them cut just a bit smaller so the students have a bit more wiggle room.

I will not be ordering any until next year,(which is almost here) but will get back to you David, sometime in Jan.

without double checking when orderning more than 24 of one size the price is about 7.20 , this does not include the back board nor of course the s&h, which has always been reasonable.