how to avoid being manipulated into spending money you didn't intend to?

Make a list of the "extra" stuff you need and/or want. Another bookshelf, a pair of summer shoes, Seanan McGuire's latest book, dinner at a fancy restaurant... and don't buy anything else no matter how tempting an offer you get. Set aside/save up money for the things on the list ahead of time, and once you have enough, start looking for deals. Don't buy on credit, even if it sounds like you get the items cheaper that way. Interest is good when you get it, not when you pay it ;)