senlinlang

For temporal method expenses related to NonMonetary assets are found by translating at the same rate used to translate the asset

so for COGS you translate at the same rate used for the inventory sold. For companies that buy their inventory over time this would mean using many different historical rates (not just one average historical rate; though as a sidebar an ave. rate can be used under certain circumstances). Depending on whether FIFO or LIFO is used you would use those corresponding exchange rates.

The easiest way I've found is just writing out on a piece of paper the inventory that was sold, exchanging each piece individually and then adding them together.
Like this:

____Inv____xchange rate____COGS
CAD 1000____2.00_________USD 2000
CAD 500_____1.50_________USD 750
CAD 2000____1.00_________USD 2000

Total:
CAD 3,500______________USD 4,750