Low strength spellcasters could use a strong, loyal companion to carry any loot and excess equipment. They can fairly easily get a loyal companion in a familiar, but most familiars aren't exactly weightlifters themselves. How can I best optimize a familiar to haul and caddy items for me? I'm especially (though not exclusively) interested in lower levels, as eventually there are other solutions to carrying things. Having a familiar who can more easily find and pass items to me is also helpful.
Get a pack-pig
As a small quadruped can carry 1x the list value in the carrying capacity table, so up to 38 pounds at full 30 feet speed, and up to 115 pounds at 20 feet speed.
A goat works even better, with Strength 12 it can carry 43 pounds at 30 feet and up to 130 at 20 feer speed. (thanks to @Leezard — I had overlooked it)
You need a Helper Monkey
Sure, at low levels that pig or goat may look like the way to go but take a moment, think of the monkey magic you could have!
Armed with a tiny backpack, your little buddy can sort through a 5 pound collection of wands, potions, and scrolls, 15 pounds at a slower movement speed. That's a lot of scrolls. And those little monkey hands can pull stuff out and pass it to you in your moment of need! Again and again, your Helper Monkey will show you it's not about how much they can carry, it's about how they can help you use it.
Monkey is the best of caddies!