You select the sizes of the two mats from two drop down lists, and depending on the size selected for each mat the price will change. At the moment the smallest mats are showing as sold out, so you'd have to select at least the next size up (18 x 24 inches) at an additional cost of $6 each.
If you want just one large mat your total cost will be $18.99 (drop price) + $245 (48x96 mat) + $6 (18x24 mat) + shipping (Free if you're in the US).
Just click on the Join Drop button to see all the details for yourself, it will take you to a checkout page where you can select options and see all the pricing. It won't commit you to anything unless you fill out all the payment details and click another join button. Massdrop really should rename the Join Drop button to something that sounds less final!
PS. I'm not sure what the actual size of the largest mat is, It's listed as 48x96" in the description, but 40x96" in the drop down list in checkout. One of those is obviously wrong.