Why would you assume that Costco returns are due to supplier mistakes?
Costco are legendarily permissive with returns, to extent of things like accepting bare stick-like xmas trees back after xmas, and giving a full refund, but ultimately this is to their advantage in encouraging mindless consumerism (which is also the general American model - no-question-no-fault returns are generally an American thing, not a worldwide one).
Now, a liberal return policy may work out for Costco, and Costco is obviously a high volume hence desirable customer for a supplier, but if Costco is pushing much of the cost of returns back to the supplier, that does change the picture a bit!
Those returned trees don't get sent back to the supplier, they get deducted from a pre-negotiated spoil allowance which is something separate. The supplier returns will be things like badly stacked palettes.
Costco are legendarily permissive with returns, to extent of things like accepting bare stick-like xmas trees back after xmas, and giving a full refund, but ultimately this is to their advantage in encouraging mindless consumerism (which is also the general American model - no-question-no-fault returns are generally an American thing, not a worldwide one).
Now, a liberal return policy may work out for Costco, and Costco is obviously a high volume hence desirable customer for a supplier, but if Costco is pushing much of the cost of returns back to the supplier, that does change the picture a bit!