Is this legal?

I got a postcard in the mail about a sale at a store that I shop at. Now this store carries both regular and plus size women’s clothing, but the plus sizes are not sold in stores, only online. The sale went as follows: on such and such date, 40% off your in-store purchase, and on such and such date, 30% off your online purchase. Considering one could never buy the plus sizes in the store, is that some kind of discrimination? Shouldn’t the people who buy the plus sizes be entitled to 40% as well? Because the store doesn’t stock them in store, that shouldn’t be grounds to not include plus size women from an extra 10% off.