Question on dealing with contractors

[Ruby the Liberal]

I have one currently working on my house who is giving me bids as he goes and is fixed price. Reasoning for this is because of hidden damage that couldn't be quoted until demolition was done (yep - I had rotted floor joists).

So in one of these bids, he cites the shower repair as "install shower base" and at the bottom, it says "Labor and material cost $2,200"

That was dated April 6th. On March 5th, I gave him a check for $1,000 to buy the shower base because of the lead/shipping time.

Now, he is telling me that "oops", he forgot to include that in "materials" in the $2,200 bid because I gave him a check in advance.

Questions:

Any recommendations on how to handle this?

Does $3,200 even sound reasonable for a shower base, drain, greenboard, membrane and plumbing behind the walls? Quote specifically does not include faucet and tile work. This is a 4x5 walk-in shower with no door.

It is reasonable to ask for receipts from the "materials" on a fixed bid project where something has been claimed as being missed?

Thanks in advance for any suggestions!