The games really fun and cozy, I really enjoy it!! The art is beautiful and it crates a relaxing atmosphere, from it's perfect art to it's calming music. However, I have some opinions on things that could be improved, mostly for the endless mode. (Sorry in advance for the long text)
Fistly, the loan payment may sound scary at first, but 7 days is more than enough to save up $10. I suggest making it higher or make it grow gradually, because once you get to the second payment you're already able to earn $50 per day.
The next thing I think could be added is if the packages could be "stolen" or dissapear from one day to another if they're left outside the locksmith, like stamps do if you leave them from one day to another on the desk. This could add a "lost packages" fee after a day and also make the locksmith more useful.
Lastly, I feel more events or side quests like the stamp collection could make the game more fun and dynamic. An example could be quests like sending x amount of packages on each route, and an example of an event could be different climates that make more or less people come each day, and these could be predicted with the newspaper. Or another one could be getting different sets of routes each week, putting to use all the nautical flags in the book
These are just some suggestions, so they aren't 100% necessary and the game is already beautiful itself. But I'll probably leave it once I finish the stamp book (or when I get frustrated for not being able to).
Sorry again for the long text and thank you for reading!! (and if you're the dev, thank you for making the game :D )