I have used Rover North with great success for many years dating back to the mid 90's. They almost always have the part you need but they are never the lowest price and the shipping is always on the high side. I needed a freeze plug last year and because I couldn't find the size referenced anyplace I ordered from RN. They delivered quickly but charged $10 for a small soft package and I paid full retail. I have ordered many small parts lately from the UK and the shipping has always been quick and cheap. The price for the part has also been lower in most cases. I always try to shop US first then if I can get it lots cheaper I go overseas. I have also used a couple of eBay sellers with good results all based in the UK.
Shipping anything anymore is just unpredictable and expensive.
I shipped a small box (about 3" x 3" x 7" weighing 1lb to Michigan from Philadelphia. - $17
Shipped another box (about 7" x 7" x 11") weighing 25lbs to Miami from Philadelphia - $20
And that is BEFORE any S&H charges that might be added depending on what I ship.
UPS likes to charge for volume AND weight. Sometimes it sucks and sometimes it really sucks.
Years ago freight companies added a fuel surcharge to invoices because of increased fuel costs. Now that the fuel costs are back down to the level that they were before the surcharge they are still charging the surcharge. Good for them when the surcharge could pay for the entire load of a tractor trailer and you only have 2 skids on the truck.
But comparing shipping costs and time between domestic and europe can almost be the same or quicker.
I do try to spread my spending around though.