Shipping methods not acting as expected

We sell books and ship in both large and small quantities. I have configured shipping using table rates and the Woocommerce Conditional Shipping plugin to limit shipments under a certain weight to Priority Mail only. Any shipments over the threshold weight are to be shipped by Fedex only. When the customer enters a small order everything works as expected – the only rate they see is the single Priority Mail rate. If, however, they go to check out and for whatever reason use the back button on the browser and return to the shopping cart all of the rates – Priority Mail and Fedex – all appear in the shipping section under Cart Totals. The Priority Mail rate is selected but often times the cheapest Fedex rate is cheaper than the Priority Mail. I’m not sure we have that many customers who do this but I would like to have the shipping in the cart totals on the cart page show only the Priority Mail rate and not all of the shipping rates if the customer returns to the cart. I believe that it has something to do with the cookies but I’m not sure. Is there any code I can use to fix this?