It is heavily dependent on the bid/BIN price of the comic...
A low-moderate priced ($4.00 to $50) comic definitely not worth buy through GSP.
A rare NM comic that costs $100 or more... maybe worth it. Depends on total cost, including the GSP costs.
Try to compare vs other shipping methods - and remember - other shipping methods may depend on you forking $$ over when the package is delivered.