Have you made full use of the shipping settings in your shop, to make sure the buyer is getting charged the right amount, and to make it easy for you to update your shipping costs when you need to?
Shipping Profiles: You can create shipping profiles under the 'shipping settings' option from your shop manager. Create a different shipping profile for things depending on how long they take you to make and dispatch, and how much they will cost for post and packaging. For example if you sell prints which only take a day to print and package, and can be sent in a flat envelope, you'd create one shipping profile for that - name it 'Prints'. But if you also sell a handmade wooden trinket box, which takes a week to make and would need to go as a parcel, that would need a separate shipping profile - name it 'Trinket Boxes'.
Then you can select the 'Prints' shipping profile EVERY time you list a print, and the 'Trinket Boxes' shipping profile EVERY time you list a trinket box (as long as they weigh about the same and are roughly the same size - if not you might need a different postage cost).
By using shipping profiles instead of entering the processing time and postage cost individually on each listing it will save you TONS of time when you need to update them. For example if at Christmas if you need 3 extra days to make the prints, you can just go in and change the processing time on the 'Prints' shipping profile, rather than to change it on each print - if you have 30 different prints listed in your shop you'd be there forever changing each one! It's also handy for when Royal Mail put their prices up, as they did recently.
Here's the Etsy link for more on Shipping Profiles: https://www.etsy.com/uk/help/article/190
Shipping With Another Item: When you add a shipping profile it will ask you for a price for shipping 'One Item' and a price for 'Each Additional Item'. Buyers will always pay the 'One Item' price. Then if they choose to purchase another item from your shop they will ALSO pay the 'Each Additional Item' fee. So this fee should only be the difference in price between the 2, not the same postage fee again. Try weighing a few combinations and working out how much more it's going to cost you. WHEREVER POSSIBLE you should make the 'Each Additional Item' cost FREE. People are more likely to buy more items if it doesn't cost them any more to ship it. This isn't always possible with heavy items, but make it work where you can!
Here's the Etsy link to discounted shipping for multiple items: https://www.etsy.com/uk/help/article/191
Shipping Upgrades: Finally, you can offer a 'shipping upgrade' option for your customers. For example they might want to pay more for one day delivery. You probably shouldn't add this on items which take a week or more to process though - if customers see the option for one day delivery they probably think it's going to arrive super fast, and not read the processing time correctly. It is great to add it onto items that you have in stock or can process quickly though. Also if you sell to the USA, and normally send by International Standard Post, you might want to add a shipping upgrade in case the customer would like their item to be tracked. See here for more info on Shipping Upgrades: https://www.etsy.com/uk/help/article/4414