Oh that sounds good!
I buy from a number of different wholesalers - Longforte mainly, although I do lots of mugs, coasters, clocks etc so I don't know if they'd have what you're looking for. To find wholesalers I really just typed mugs wholesale uk into Google, and went from there!
I ship through Royal Mail, and always tracked, usually Tracked 48. I buy labels online and have a thermal label printer as it's so much easier that hand writing addresses and getting a label from the Post Office.
Etsy fees are quite complicated, but this will give you a good idea - scroll to the bottom for fees on other sites - https://www.ebayfeescalculator.com/uk-etsy-calculator/
I set us as a business on Ebay. I know there are people who don't, but if you are clearly selling stuff to make money, they could close your account if they realise. I wasn't willing to take the risk. It's around £30 a month, their help pages are really good so you can read all about it there :)
Amazon Handmade is Amazon's attempt to capture some of Etsy's business. The main thing is that you have to apply and be approved to sell on Handmade. If you don't get approved you could always sell on the main Amazon site if your products are suitable - I can't advise on that though as I've ever done that.
The main thing I do is send everyone who buys from me a business card with my website address on, and everything is a bit cheaper there, so hopefully repeat buyers will come there instead of Etsy Ebay and AH - fees are cheaper there so eventually I'm hoping most of my sales will be via the website. It's worth waiting to open one though until you know if your business is working, as initial setup is quite expensive and you need to know you'll have the customers.
Hope that all helps - please ask if I've made anything sound really confusing!