Thanks for all your advice guys, it is much appreciated. I brought it in to another bike shop to ask about the longer seat post solution, but he pointed out that my bike can only take the quill seat posts because there is nowhere to tighten at the top of the tube.

I've been trying to upload a picture all week before eventually contacted an admin, but I got told I can't because I haven't reached ten posts yet (can't post URL to other website with pics either). So all I can do is describe. It's basically a bulge in the tube that covers the area where the seat post is. Consensus seems to be to keep riding until problem gets serious, then chuck it. I'm lacking other solutions and the repair cost does seem expensive, so it seems like I'll go with it.

Thanks again for all your help!

As a final question, are there any other types of bikes you'd recommend? I like the aesthetic of 80s road bikes, happy with not the best quality, just something that can take the daily commute for years to come.