For the record, I have never had a trainee. If I did, I wouldn't treat them this way. Being a slow-pay or a deadbeat is no way to engender long term realtionships or a good reputation. This attitude will cost your boss a lot, sooner or later.
The preceding paragraph was a disclaimer for the balance of this post. I'm normally a very strong advocate for appraiser-power. But that doesn't necessarily extend to trainees who are trying to get into the business. There are a lot of people who would like to have the opportunity you have right now. There are more ways than one to benefit from your current relationship. Maybe you should consider just making the best of it. Only you can decide what is right for you.
The truth is that some fee shop operator/appraisers are not very good business people. The last state newsletter that came out in my state listed at least one appraiser who had been disciplined for not paying their subcontractors. I worked for an appraiser once who paid all their bills and invoices slowly, took their ledger with them on weekends and basically did everything you are describing. It never changed the entire time I worked for him, and I reckon he still does it that way to this day. However, if I had it to do all over, I probably would still have gone to work for him because I did get paid for (almost) every job I ever did for him. He did stiff me for a few jobs he never got paid for, and many times the payment would come 90 or even 120 days after I completed the appraisal. On the plus side, I learned a lot at that shop and benefitted in other ways than immediate income. To this day, I consider the whole experience very worthwhile as a whole.
The one thing I did do was to hook up with another fee shop. That helped a lot because it stabilized my income. When one shop was slow the other one usually picked up the slack. Also, the two shops knew I was doing work on the outside, and they tended to not take me for granted. One of them even offered me a 70% split to get me to 'commit'. I was smart enough to not take it. Having options is a valuable commodity.
It probably won't do a whole lot of good to threaten your boss. It's a small world out there and no trainee needs an enemy. If you can't abide the way things are now, you should probably just move on with a smile on your face. Under no circumstances should you bad mouth them to other appraisers for being a slow-pay or a deadbeat. Besides, their reputation is probably already well known. 18 assignments completed probably means at least 110 hours, so you are that much closer to having your permanent license. Burning bridges might be satisfying on a short term basis, but could come back to bite you later on. As a person who needs the documented hours for licensure, you don't have much leverage right now. Better to pay your dues right now and benefit later.
It's also better to get paid part, or in pieces, rather than take the whole loss. Maybe you can make arrangements that if your boss is strapped for cash that they can pay what they can when they can, with the understanding that you eventually will get your money. Being flexible sometimes can help. Whatever decision you make, live with it. There's no point in being in a position where you feel like you're a doormat. You wanted to get into the business, this is sometimes what the business is like.
George Hatch
My son would tell you the BEST supervisor is your DAD- we pay our Trainee (son) before we even get paid- I am a real estate broker too and agents and appraisers have families to feed too, so my advice is Find another sponser- in Texas you can have several -