Quick thing I wish someone had told me before my first dev hire: the quote is not the cost. Not even close. I figured that out around month two, which was not fun.
The quote covers the work you described. It doesn't cover the stuff you didn't think of, and that's the list that matters:
The single best habit I picked up is asking, before signing, "what's not included?" It makes people squirm a bit, but you get honest answers. A short paid trial helps too.
When you hire dedicated ecommerce developers, I'd also look for someone who'll stay on after launch rather than hand over the files and vanish. That one thing saves more than any discount on the rate.
What about you? What cost caught you off guard?