Hi Lil Dot,
Copy the formula in cell G2 and copy it across and down to sum numbers based on both on months and customers.
=SUMPRODUCT(($C$2:$C$6),(--($B$2:$B$6=$F2)),--(MONTH($A$2:$A$6)=VLOOKUP(G$1,{"Jan",1;"Feb",2;"Mar",3;"Apr",4;"May",5;"Jun",6;"Jul",7;"Aug",8;"Sep",9;"Oct",10;"Nov",11;"Dec",12},2,FALSE)))
You will have to expand C2:C6 ranges in the formula to your desired range.
Kind regards
Saba