Listen, my uncles a millionaire, and he didnt get there by spending money on dumb shit all the time. Just, ya know, some of the time. Your MIL probably knows exactly how important it is to save when and where you can. My uncles favorite thing to do is shine on people he loves and cares about. He loves to be generous when he can. I really wouldnt worry about offending your MIL, you probably made her feel useful and good.