Thanks! Blatantly ripped off the code in the NewsListing snippet and popped it into the ShopCart snippet (well, I used my favorite ternary operator, but it’s the same logic).
$furls = $modx->config['friendly_urls'];
$char = $furls ? '?' : '&';
At this point, the TemplateSwitcher links are hand-coded in a chunk, so I need to remember to use whichever character is appropriate for the site in question. Eventually, however, I’ll have a snippet to automagically generate the links from the templates available, and this code will come in handy.