I'm posting my solve for anyone else dealing with the same issue. Even though the data extension field was set to Number and I was able to access the value with %%=v(@Pens)=%%%%=v(@Pens)=%%
I was getting errors on any math function.
All I did was change the field name in the DE to "PAPERMATE PENS BLUE 15ct"PAPERMATE PENS BLUE 15ct
and update
SET @Pens = AttributeValue("PAPERMATE PENS BLUE 15ct")
SET @Pens = AttributeValue("PAPERMATE PENS BLUE 15ct")
It's working now.