$5 is for stamps and small items that fit inside a 6x8” envelope. If the order is large enough (e.g. two postcard sets) to require a more expensive shipping it’s simply viewed as a bulk discount for the customer. Average expense for me is $4.6-$5.15 for domestic shipping but can go up to $27.15-$31 for international orders, excluding the expenses of packaging materials.

$10 is for posters and comics. International shipping costs on average $12.5-$14 (sometimes as high as $24) at the post office, while domestic shipping costs between $5.5-$7.5. This excludes the expenses of the packaging materials, which cost between $0.81-$3.26 each. This was therefore as low as I could possibly go with the pricing.

$20 is to cover anything that requires more extensive weight or packaging.