The shop should be able to switch out the cassette to a SRAM 12-28 for essentially free. Shimano road rear derailleurs are rated for 27, but they work okay with a 28-tooth cog.

As to the compact, I think it offers a very versatile setup. I run a compact crankset and a 12-25 and I'm a hill slug. For my commuter I used a triple, but for regular, unloaded road riding a compact is going to do the trick for most folks.

It would be quite costly to switch to a triple crankset. So costly that I think you'd be better off looking for a bike that already has a triple if you really want one.