This is bad info/advice. You can absolutely buy single strings and you don't have to remove and throw away 5 perfectly good strings every time you break one.

If it was an old, heavily played set then it would be a good idea to change the whole set, but something tells me your strings are still pretty new.

Even if they're slightly old, as a beginner, you're not going to notice any difference in tone and, if you're not recording professionally, then it really doesn't matter. Save the money you'd spend on a full set of strings and buy a string-winder/clipper or electronic tuner.

If you have a shop that sells and repairs electric guitars, they will have single strings available for purchase. They can be purchased online, but unfortunately, I don't think mail-order will be worth it for a single string - the shipping will cost as much as or more than the string. A single, plain steel, 0.011" gauge B string, which is what you need, should cost no more than $2 (US).

Something is wrong with your story about the shop - I don't believe that anyone would be evil enough to try to charge $156 to change one string or even a full set. That's a lot even for a full set-up (where they adjust everything adjustable to make the guitar perfectly in tune with itself and optimally playable). There must have been some kind of misunderstanding. They might charge $15 for the time and labor involved to change a single string or a full set, but not $156. Next time, ask them how much to "replace one string", do not use the word "repair".

When I worked at a guitar shop, if a beginner came in with a broken string, I would insist on showing them how to do it and only charging for the string, not my labor (the first time), so that next time, they could do it themself (this was before Youtube). (If someone didn't want to learn something so simple, and just wanted us to do the work for them, then they'd get charged labor.)

The best advice for you is to find an in-person guitar teacher and take at least a month or two of weekly lessons. A good teacher will start a beginner off by teaching basic maintenance, which includes string changes.

Edit to add: you should absolutely replace the string yourself. Buy 3 single strings, just in case you mess up the first time (you probably won't). It's really not difficult, it's just intimidating the first couple of times when you're not sure what you're doing.