As such it is a little different from server based systems. I'll assume that you and readers of the list will be interested in the technicalities and can understand javascript even if you don't know much about on-line shopping.
Main file is a frameset with a lot of script.
There are several invisible frames and one full-screen visible frame which is for the shop designers to use as they like including making a further visible subdivision. Functions returning names of which other files to use - which may involve conditionality. May be a simple number, or a delimited string. The sales tax version has fewer modes. I have a method for setting up for US Postal Service from any given starting zipcode, making use of the on-line information from USPS to create the required data relating other zips to charge zones.