Excel 2007 : Need help Vlookup, and IF function

hello all,


I am working on a project where I have invoices that I need to calculate interest at 1 day past due.
I did get a working formula to calculate days past due
I worked out a formula if per annum rate is 18% for example,
My problem now is that my IF function of IF invoice is 1 day past due to add the correct amount of interest per day
I keep getting an error.

Since we have several customers I was trying to figure out a way that on another sheet in the workbook have a list of accounts and their annum rates to calculate in the spreadsheet,

Then on the last tab make it so that even if a payment is deducted a running true balance will be there.
once completed my boss would like it to be a Macro so that others can use it calculate interest on invoices past due.
Thank you