What I can't figure out though is proof of postage.
Website shows that certificate of postage is given as part of registered mail. But not Standard mail.
Can I still get a certificate of postage for standard mail? Even for an extra cost?
Proof of postage is meaningless, just a receipt.
I do not advise using An Post for registered mail. Outside Ireland it is extortionate and in the event of lost mail etc good look getting compensation (via their insurance).
You do run the risk of scammers and it can happen. I had one this week, a German buyer who opened a Paypal claim 4 working days after I posted the packet. Paypal decided in her favour. That said, this is (to date) super rare. Most people are clearly honest and you will not likely attract much buyers if you charge EUR 10 instead of EUR 3 for posting about 100g of parts.
Royal Mail may have a more cost effective registered post (most of Europe does) but generally here it's a flat 7 euro or so no matter whether you are registering a letter or 3kg parcel. Quite ridiculous really.
Happy to share experiences if you've more questions pop me a PM.