I still vote with you that they should be supplies, even 'maintenance' supplies, if you choose. It is curious, though, that the vendor seems to agree with the client by allowing the sales tax exemption. Perhaps a chat with the vendor, not revealing the client, would be helpful as to why/how the vendor thinks this is correct. After all, the vendor must undergo occasional sales tax audits, right? Maybe there is a quirk in your state law for truck supplies. Maybe check with the Sales Tax Division at the state level?
I still vote supplies with your argument that they are consumables.