SUMIF Issue, how to not count specific values

Hi

I was wondering if you could help me as I am a bit stuck with a SUMIF formula for the attached spreadsheet.

I need to add up some figures from the 'Main Table' and show them in the 'Results Table'.
I have popped some figures into the 'Result Table' in red to show the results I need, but I need this to happen automatically using a formula.

The formula I am currently using is the SUMIF formula and I have put this into the 'Results Table' on the line below to show what I get back at the moment.

I am trying to get the formula in the 'Results Table' to count the figures for each period (P1, P2, P3) when the letter 'TA' is put into the 'Source' column. My issue is that I only need the first instance of a sale recorded, so if a figure appears for that customer in a previous period, then I don't want it to add up in the later periods, so for example in row 6, customer Beta, there are two sales. One in P1 and one in P3. I only want the figure in the first sale P1 to be picked up and counted by my formula and not the sale in P3.

Does anyone know how on earth I can make this happen without having to drastically change the spreadsheet layout?

Any help would be very much appreciated.
Thank you
Debbie