The waiting time after sending your first invoice is an emotional roller coaster. I will not promote.

I have a small humble startup focussed on helping Al builders launch their websites/apps. My usual clients are SMBs and the billing is fairly simple - defined scope - flat fee. I recently signed up a client that's in 4 figures for 1 time engagement of 2 weeks and I sent the invoice for an advance. I know it's going to come back with the advance but the waiting after doing the initial consulting and hand holding and in this phase is killing me. There's excitement and a lot of nervousness (unfounded, I know).