IF(SUMPRODUCT(MAX formula using R

I have the following table in Excel and formula in Column D and I'm trying to figure out how to execute column D using R:

Excel table

My formula in column D is the following: =IF(B2=0,SUMPRODUCT(MAX(($A$2:$A$11=A2)($C$2:$C$11=C2)($B$2:$B$11))),B2)

This formula says: If Unit price = 0, then: return the maximum Unit price of all other sales to that same customer and of the same item. If unit price does not equal zero, then return same Unit price.

Thank you