Not sure what you read or where you read it, but designers don't pay any portion of the shipping, and it has no bearing on our royalties, either.

We make a royalty on the sale price of the item after any discounts/coupons applied by the customer.

That royalty can be further reduced if the sale was referred. Referrers make 15% of the sale price after discounts/coupons. Zazzle pays most of that, and designers chip in 20% of their royalty toward it.

If you set your royalty at 15% or higher, your royalty is reduced by a transaction fee (around 5% of the royalty in the one test I've done.)

Exceptions:

Zazzle Heart products are capped at a royalty of 5%

Upcharges: There are a number of things that can result in an upcharge to the customer. Vivid printing on apparel is one. Some sizes on apparel would be another. Sometimes there are things offered with the product as an add-on - like a base for a photo block, frame for a tile, etc. Those would also result in an upcharge to the customer.

Designers only earn 5% on any upcharges, regardless of our royalty settings.

ETA: As for makers, what I'm reading states that Zazzle provides them with the shipping labels, based on package size and weight. If there's a discrepancy, they have to work it out with Zazzle.