Another option is to get the proper crank you want in a 50/34 combo as just going to a 52/36 isn't much different than what you have. Sell the barely used crank you have to recoup some of your losses. Seems to me like the cheapest option plus you'll have the gearing you want although I don't know too many people who can use an 11t cog even with a 50t chainring but you can change that when the cassette wears out. I'm a little curious about why the shop recommended the 53/39 combo unless you convinced them you were a pro racer.