With 3D printing sales and occasional bulk imports of board games I send 4-5 packages a month, and sometimes larger batches. There seem to be two main ways of achieving this easily.

Royal Mail

"Click and Drop" lives at parcel.royalmail.com. (IPv4 only, SSL Labs rating A - no HSTS, limited forward secrecy, otherwise pretty good.)

There are three ways to use it: anonymously, with a "PayPal" account, and with an OBA (Online Business Account). The PayPal account has no ongoing costs or minimum activity requirement, and that's what I use. (You can pay by credit card through this interface without a PayPal account, though it's obviously a bit clunkier.)

The system encourages you to "connect your eBay account", and implies that the only way to use the thing is via eBay; but I know this isn't true, since I don't have one. So once logged in, Orders → Manage Orders → Create New Order and enter the destination address. Then you can create the shipment to that address based on the actual package size and weight (options are presented based on the address and weight), and either pay for it immediately or (if sending multiple items) enter more orders and shipments and pay for them all at once. You don't get to re-use an address for a later order, but for the UK there's a pretty decent post-code lookup (even if it does put Bristol in Somerset). On the other hand there's nothing like the CSV import that MyHermes offers: you have to enter one address and item at a time.

(I use a Smart Weigh postal scale to get the package's weight, 2g increments to 50kg, and an acrylic size guide to check whether it'll fit into a Large Letter format.)

What you get at the end is a PDF with the address label, and (if you turned it on in your account settings) a proof-of-posting certificate on the same sheet. If you have a general-purpose computer, you can use my fixup code to add an order number, return address label, and/or customs form to the PDF before you print it.

Then take the package and the proof-of-posting to your local post office (about seven minutes' walk away in my case), hand them over, get a stamp on the PoP and leave.

So far every package I've sent via this method has arrived. Maximum shipping weight is 2kg.

MyHermes

MyHermes has a less-impressive site (IPv4 only, SSL Labs rating A- for weak intermediate certificate, no HSTS and no forward secrecy). It also does its best to hide the actual costs until you've committed yourself to sending.

This system does offer a CSV import, though the fields it uses aren't the fields it claims to use; I find I can just about get the address through reliably, but anything else usually has to be re-entered.

The printed output needs to be cut apart and one section put inside the package, so don't seal it before buying postage.

Failure rate on these packages has been about one in six, and the nearest dropoff point is slightly further away from where I live (a stiff walk up a hill, or drop in while driving to somewhere else). Maximum shipping weight is 15kg, and for substantial parcels it's much cheaper than Royal Mail; however, many of the things I ship can fit in the Large Letter format (maximum 1" thickness), for which Royal Mail is cheaper.