Early on in my current campaign my players were sent on a quest by a wizard friend of theirs, he gave them a sending stone so he could keep in contact with them. After that quest ended my players got a nice big downtime, 1 month. One of my players, who owns a tavern, asked to dedicate that downtime to finding some more sending stones, one for each player and the pairs to be held by the barkeep NPC she employs. I rolled on the tables in XGtE and got a price that they could afford.

Are there any unforeseen downsides in letting them spend all their money on sending stones? I know this effectively gives them party wide telekinesis but since they're using this NPC as a telephone switchboard (literally how they pitched the idea) I can reserve the right to say he's busy and can't forward their messages.

I decided to give them the stones and then ran a session, they got separated for a few minutes and spent most of it talking through that npc to each other instead of trying to solve the problem that separated them. They've implemented a rule that he needs to write down what they say and relay the message exactly. 10/10 it was quite funny. Try doing this with your players.