Easy work for $50, Need help ASAP

I'm switching my company over to paperless invoicing via laptops this year. It's a seasonal business and I've just learned of filemaker not to long ago and I'm starting to get busy again. I'm using the invoicing solution based on the VTC.com tutorials by John Osborne. Anyways, I've modified the solution to my needs and I need someone to incorporate a few features that I've been having trouble with and I'm basically out of time trying to figure it out on my own.

Naturally I will provide my current solution with admin password so you can modify it.

This work looks easy to me and will probably only take 20 minutes to accomplish for an experienced user.

Items needed:

1.) Right now the solution only has a drop down box to mark an invoice paid. I would like to be able to accept partial payments and payments that can be applied to multiple invoices as well as show the current balance for that customer

(I have some tables already setup for this but this was past my current knowledge)

2.) When Invoicing I would like the "pool opening" product price to automatically change based on some parameters. The price should change based on.

Pool's total square footage ( I have a field for this)

pool's type of cover (water bag covers and vinyl pop-in add $10 to the price)

above ground pool's have a standard price of $189 (unless they have a water bag cover then they are treated as in-ground price)

(I also have some tables setup for this but haven't gotten to incorporate them.)

3.) When Invoicing the "cleaning" product price is based off of the field in customers table called "pool_cleaning_price"

I can offer $50 via paypal

Thank You,

Kevin Stempien