put the following formula in cell D2:
=SUMIF(Sheet2!$B$2:$B$18,Sheet1!$A2,Sheet2!$A$2:$A$18)
You'll have to change Sheet2!$B$2:$B$18 by your actual product range
You'll also have to change Sheet2!$A$2:$A$18 by your actual units ordered range.