Never say no. Use all the available resources at your disposal. That includes NexPart, making outside buys (set up a commercial account with your other local parts stores and pay with your pcard) and thinking outside the box.
Don't slander other places. Don't talk bad about the people there. We are all trying to make a living.
ALWAYS BE HONEST. If their stuff is late, tell them it's late. If you make a mistake, OWN IT. Humans make mistakes. Be humble.
Do not over-promise. Don't tell someone it's going to make the 9am hub run when it's gonna be there at 1:30.
Outbound effort. Don't call and tell them about the great deals you have. Call them and remind them you are there. Say hey, I haven't heard from you in a while, just checking to see how you're doing and see if there's anything I can help you with.
Shop visits. Bring donuts. They like food. Talk to your DM and TSM about it. And learn your system.