RunSignUp allows races to set up simple stores. This video shows you how to use the store to make purchases.
Making a Store Purchase
To make a store purchase, simply head over to the race page of the race and select store from the left sidebar menu or the top menu.
Once at the store you can select the quantity of items you wish to select, add another item to purchase or select any specific sizes (if offered) then select Continue to proceed to the payment page and confirm your store purchase.