Need to perform join and replace on multiple fields

Hi, 

Not sure if there is an elegant solution for this, but here's what I need to be able to do:

I have one file with invoices {Invoice} and one file with rates {Rates}. I need to be able to match the rates to the individual items on an invoice. Simple enough. I've used the Find Replace function to do this.

However, I now need to be able to match against multiple fields within {Invoice}.

Example:

These fields from {Invoice}: [Product] [Modifier] [Type]

Which map to a corresponding rate in {Rates}

It is pretty easy for me to manipulate {Rates} so that each modified rate can be a separate field matched to a product row.

In Excel, my solution for this has been to create a unique key by stringing =[Product]&[Modifier]&[Type] and then performing a vlookup.

Does anyone have a better solution? I really want to avoid doing this in Excel.

Thanks so much!