The Cargo paddle ship (mail) (steam) lists that it can only transport 45 bundles of mail.
This almost certainly is meant to be some value like 450 seeing how it is a dedicated mail transport ship. A value of 450 makes even more sense seeing how the mixed transport ships available around the same time have values such as 200 mail (available before) or 500 mail (available after) on top of passengers and have similar per km costs.
To put it in perspective 45 bundles of mail is the same as what 3 children riding ponies can move around. One would expect a 50 tonne dedicated mail ship to move more than 3 ponies worth of mail per journey...
Thank you for your report: that is indeed an error, which I have fixed by increasing the load to 450.
Forgot to bring closure, sorry...
This is now fixed. The ships now hold 450 units of mail. Although I would argue that 450 might be too low (800 would make more economic sense) I do not have any historical knowledge of the ship type to back this up and in any case it now is vaguely usable.
Splendid - thank you for confirming. If you or anyone else do(es) manage to find out historical mail capacities of ships, please do let me know so that I can make any changes necessary.