Not familiar with MyHeremes, but I'd be very surprised if there weren't a number of such private options in the UK if the royal post gets to klugey (like UPS and Fedex here in the 'States).
I've never been a seller, but as a buyer postage has always seemed a bit arcane. Even just within the US, postage for similarly sized/weighted items using the same service/class can vary so wildly from seller to seller that I honestly have no idea what's going on. It feels like each seller is shipping from a different parallel universe sometimes. Not much you can do but shrug and accept it, unfortunately.
International postage is doubly weird. I buy from the UK from time to time, and despite what's been said in this thread, shipping and transit time is usually pretty comparable to what it would be from inside the US. Dunno why.
Bought from Italy once, and despite he Italian post's reputation, it went well, and IIRC wasn't too expensive, though it did take a long time.
Bought from the Netherlands once. Was expensive (eight bucks for literally just an envelope containing a single tiny spring) Package disappeared in transit and had to be resent. Only time that's ever happened to me. Once resent it arrived like lightning though.
Tried to buy from France twice, only to cancel upon seeing the comedy postage quote. We're talking 20 euro to ship 2 figures, that sort of thing. I dunno why France in particular should have such bizarrely OTT postage, especially since they're EU and thus should theoretically be similar to other EU countries. I don't look at french companies/vendors anymore for this reason.
Bought from Oz a few times. Postage IIRC was quite cheap due to the exchange rate favoring the US dollar, but transit time was comparable to buying from Japan.
Buying from Japan is always expensive and takes a long time to arrive. Dunno why, as it's a straighter shot to California than from Europe, and presumably the US gets a lot of trade goods from there anyway, so the shipping lines betwixt should be robust, one would think.
As a buyer I like my transactions clean and simple as possible, so I generally avoid transactions where I'd have to actually email back and forth over something before buying. I've done that once or twice, but it takes a very special and very rarefied item to push me to it. I would assume vendors feel the same, so I would never waste both our time initiating such a conversation unless I was quite serious, and would never leave a vendor hanging. I just want the transaction done as quickly and easily as possible, so that would be as much a waste of my time as theirs.
I'll call a vendor with little hesitation if they have a service number, but I f****g hate the crawling inefficient slog of email "conversations". If I have to have an email convo with a vendor before buying, 99.9% of the time I won't even consider buying in the first place. Related: vendors who only sell through Facebook, or who actually have a website but only take orders via email (seriously: what is this, 1995?).
Stopped buying on Ebay regularly around eight years ago due to the then rampant rise of feedback ransoming. Dunno if that's changed much. Hope it has, but don't expect it has. Big corps make CS changes largely based on Wile E. Coyote-ish lines if reason, so when they actually do fix something, it's kind of a rare random "stopped clock" sort of thing. Still buy on ebay once in a blue moon, but only as a last resort. Every time I go there it seems like I have to wade through 2000 listings for crappy cheap Chinese "alternative" brands and outright pirated goods in order to find the 3 worthwhile listings of the thing I'm actually looking for.