It's either a scam testing to see if you'll follow through and pay it or it's a billing alert set up by a previous owner of your phone number (not your republic wireless phone number, but the underlying sprint number associated with your phone).
Either way, you don't owe the money, so just delete it and don't worry about it.
There are no charges from Sprint when you have a republic wireless phone. Your service costs $19 a month plus taxes and government fees, and that charge is billed to the card you have on file, monthly; not texted to you.
SouthpawKB, customer since 2011, not a republic wireless employee