Can you have Plastiq send a check to wherever you're buying the bonds at? This would avoid sending a check to yourself and the plastiq fee (I think).
But if you're willing to float for that long, you might want to check out Kiva. I haven't used it yet myself, but I think you'll get higher returns, and it's setup to take credit cards directly avoiding plastiq altogether. If you do go Kiva, make sure you spread out the money instead of dumping it all into one project/loan.