Net and catalogue shoppers could see unexpected sales tax charges tacked onto their purchases starting tomorrow. On Oct. 1, 13 states–with five more scheduled to follow over the next two years–are slated to kick off their participation in the Streamlined Sales Tax Project. As previously reported by News.com, the program is aimed at streamlining widely disparate state rules for sales tax collection–with the intent to encourage companies to collect sales taxes even if they don’t necessarily have to do so. … For now, the new streamlined system is voluntary for companies that don’t already have to collect the taxes, and it’s hard to say how many will sign on. But all of that could change if organizations like the National Retail Federation and the National Governors Association get their way–they’ve said they hope to get Congress to make participation mandatory.