I often come across using a lengthy formula for a data like:

Product Rate Qty
xxxx xx xxx
xxxx xx xxx
xxxx xx xxx
xxxx xx xxx

to calculate the Weighted Average Rate where the conditions are that:

1. Product is "ProductA";
2. Rate exists; and
3. Return "0" to avoid a #DIV/0! error if no such product exists.

Such a formula is:

=IF(ISERR(
SUMPRODUCT(--(A2:A10="ProductA"),B2:B10,C2:C10)/
SUMPRODUCT(--(A2:A10="ProductA"),--(B2:B100),C2:C10)),0,
SUMPRODUCT(--(A2:A10="ProductA"),B2:B10,C2:C10)/
SUMPRODUCT(--(A2:A10="ProductA"),--(B2:B100),C2:C10))

Now I want to create an add-in for a function to be uploaded everytime I
open Excel.

What would be the appropriate piece of code for a function and arguments like:
MyRate(Product, Range1[Product List], Range2[Rate List], Range3[Qty List])