Converting a large excel file into a format for their new inventory system is valuable to them. But it might not be that complicated for most developers.
They would probably agree to a fixed fee more easily than a higher hourly rate. There is some risk here if you're not confident how long this will take.
Figure out what your target hourly rate should be based on what you want to make or feels fair. Then setup a fixed fee price that works for you.
I would propose three phases.
A. Initial meeting, they provide a small sample of their existing file (a few lines) along with the same data in another excel file manually converted to the format their new software requires along with the full excel file they want to convert. Also ask for rules/requirements, special cases for the conversion.
B. Setup a sample 50 to 100 lines as an initial test, that you will convert and provide them as a test to verify the conversion is working properly.
C. Once they ok part B, convert the entire document.
So maybe part A: $50, part B: $50, part C: $150.
Prepare a contract ahead of time with the Phases and amounts included so they can sign off on it at your meeting. A simple one or two page contract should work. You can probably use the short version of the contract eschutte2 posted. Just replace the hourly rate section with a fixed fee section.
I would invoice them $250 payable by check when you deliver the final excel file(note this in the contract). Companies usually don't like writing multiple checks for small amounts.
If you do part A and part B and the conversion isn't working out, invoice them $100.
Let them know that you will provide support if there are issues with the conversion at no additional cost if it's an issue outlined in their original rules/requirements for the conversion. If it's a new rule/requirement you can provide that for an additional fee.
They will probably ask you for a W-9 tax form for their accounting department so they can send you a 1099 for taxes, required if they pay you $600+ for the year. You'll probably be operating as a sole proprietorship which is ok, I'd recommend getting an EIN (free online IRS) so you can use that instead of your social on the W-9. You could prepare this ahead of time or send with your first invoice.
I would include a clause in the contract that the deliverable is the converted excel file, the scope of work is converting the excel file and that you retain the exclusive rights to code developed to make the conversion.
Google convert excel for 'name of their inventory software' if it's a popular search/topic. You might be able to setup an online conversion tool that you could charge companies a one time fee for converting their file online. Might be a fun project for you and a potential money maker.
Good luck with your first freelance project.