Most places have a minimum shipping cost regardless of what you order. Some base it on size, others base it on the dollar amount.
As mentioned previously, the best way to avoid this is to add items you anticipate you will need in the future.
I've never had a problem with Marco. A while back I placed an order that included 2 pieces of playfield glass (one a bally wide body) and the shipping was about $10. Most places won't even ship glass.