Multiplying countif text matches within a range

Hi All,

On the attached file, in the 'stock control' tab, in total stock added table I have added a counter to record each time a stock type is chosen from the drop down lists in the first 2 tables, but cannot work out how to multiply that by the quantities entered in the next column. I've put some numbers in for 'Sensor Controller' as an example, where there have been 2 orders of 20 received and 2 units returned to stock. This should mean I have 42 units of 'sensor controller' in stock, however with my very limited knowledge of using these I can only seem to make it recognise each entry as 1.

I hope this all makes sense. Also, last time I posted a similar problem, somebody just completely did it for me - which was great, however I'm also hoping that someone could point me to some resource whereby I can learn to implement this myself next time!

Many Thanks in Advance!