Southwest is awesome for these return trips with extra Disney gear! I use the collapsible duffle regularly as a strategy to get merch home. Thank goodness for expandable checked baggage as well
Agree with others that if you see an item that you really want in the park, buy it rather than waiting for a later opportunity. Online can be very sporadic and things are hit and miss even at the parks.
Also consider taking advantage of sending items back to your room from the parks if you have a multi-day onsite stay and don’t wish to carry your purchases around the park all day. For other, more expensive items, shipping can be an excellent option.
Happy shopping!