To achieve this without a helper column:
=INDEX(Purchases!C:C;MATCH(1;(Purchases!A:A=A2)*(Purchases!B:B=B2);0))
(You will need to confirm this with Ctrl+Shift+Enter in Excel; in Google Sheets it will work "as is" due to the fact that the INDEX function itself enables array calculations.)
Or a Google Sheets-specific solution:
=INDEX(FILTER(Purchases!C:C;Purchases!A:A=A2;Purchases!B:B=B2);1)