I am working on a boating services website in Squarespace and have selected Foxy Cart as our check out system as they are using Authorize for their payments. I am a web designer so a lot of the threads and topics here are hard to follow but I think I am getting most of it so far. What I need to do is have the user put in their boat length into a field on the checkout screen and have that value multiplied by $15 and added to the end amount. Is there an easy way to do this? I have created the category for this but am not sure what to do from there. Any advice will help. Thank you!