The first 2 crates in the train ask for a maximum of 50 of a specific product.
You have to fill those first 2 crates and the other crates to their max capacity of 100 ONLY if you want to earn the Complete Shipment Bonus.
If you want to sell products just to get some Coins, you can just ignore the first 2 crates, and put whatever you want inside the rest of the crates, in a quantity of your choosing — you will still get THE SAME COINS AS SELLING FROM THE BARN as soon as you add something to the crate.
For example, if you want to send a train with only 31 Wool Spindles in a crate, you can do it without any issues.
When you click “Add to Crate”, the 4,960 Coins immediately go to your Coins total at the top-left.
If you don’t want to deal with any more crates or sell anything else, you can just send the train away.
Then confirm you want it to go away with an incomplete shipment, and that’s that.