This is very similar to, but almost strictly better than, Instant Summons, which is 7th level. The only thing Instant Summons can do that Word of Retrieval can't is get something from another plane.

Compared to Instant Summons, you have:
greatly increased the weight limit
removed a 1000 gp material cost
made it require a save rather than automatically failing for attended objects
made it non-dispellable
removed the ability to cross planar boundaries
made it take up a spell slot until used

I don't think the two drawbacks I listed are enough to counter the four major advantages. This spell should be at least level 8.