Why not? They get a commission. Banks issue close similar deals (letters of credit) all the time for single-container shipments in international trade. A 20K transaction is certainly not below their notice, they have entire departments specialized in services for small business. The less trade volume you have with them, the higher their commission (in %)!
Not sure what gives you an impression that there is some minimum amount for a trade finance deal. Bank guarantees are a form of trade finance, and you have other simple instruments. A google search brings up 2 or 3 providers who would do an online quote for amounts starting at $10k.
Structured trade finance is a whole other matter, that is where you have bespoke instruments and teams working on deals - usually with businesses that see $50M+ p.a in deal flow.
In either case I mentioned in my post that having a lawyer do a terms of trade or supply agreement is cheaper at the low end.