I had pretty low expectations (just with online helpdesks in general), but TheKnot has EXCELLENT customer service. Everytime I had a question, someone responded via email within 20 minutes with a detailed explanation on how to do whatever I was asking.
If you live in Canada, and can't do a cash registry, then they let you make a hyperlink to an alternative site! They're super helpful and everything I asked, they had a solution for. I don't usually write reviews, but I was very pleasantly surprised with The Knot.