I don't think one more week would have made that much of a difference (rather than, say, a couple of months). Seems like they came up with a solid solution anyway.
This is complete speculation, but I'm guessing that the Starbucks deal happened late (as does any deal between higher executives that don't have to implement it), and the dealmakers wanted it running as soon as possible.
Actually, Postmates & Starbucks have been working on this partnership for a while now. We began operationally testing this in the last couple of months. As with all tests, there were kinks that needed to be worked out and new problems to be solved. Transporting coffee, spill-proof, at scale was one of those.