I think the that the biggest factor you can change is your packaging.
It’s really easy to buy or get packaging that is a little bit bigger than what you’re actually sending, like if you’re using a box when you could be using a poly mailer, or even a padded envelope if you need a little bit more protection. Your shipping cost is dependent on the size of the package and the weight of the package. Cardboard is heavy, and anywhere you can trim down weight will help you win over time
If you are paying for your packagings, make sure to ask your carrier for giveaway free packaging. USPS has poly mailers and boxes that they just give away for free. They’re for specific mail classes, and sizes, but it’s worth taking a look to see what you can get for free.