Judging by the packages I regularly receive, it seems few people understand the differences between the various shipping carriers. Here are my experiences.
First, international shipping. I have given up doing carbs internationally. Shipping is the main reason. At one time USPS worked well. It's low cost and ease of use was a boon, offsetting the historic quirkiness of the service. But no more. I have been ripped off (literally) too many times and dealing with them is self-abuse. If I do any more carburetors internationally, it won't be through USPS.
Speaking of USPS, though it happens fewer than one out of every ten packages, I am amazed every time I receive carburetors from customers shipped USPS. I mean really amazed. The US Post Office is virtually the posterboy for bad service. They're inefficient, unresponsive, and expensive. They're so bad I use USPS only when for some reason I am forced to. Ever made a claim? Good luck! USPS is famous for not taking any responsibility. And if it is an international shipment, give it up. The first thing you will hear is, "Oh, you have to talk to US Customs about that." If US Customs has a customer service department, I have never been able to find it. USPS will even keep your money despite having returned your shipment to you, if it comes to that. During Covid, Australia Post refused almost every booklet I sold over there, and USPS returned (some of) the booklets, and kept (all of) my money. Carburetor shipments to Norway have also been returned, and again USPS kept my money and would not explain why. Then there is the expense, even domestically. Once you exceed 5 pounds, USPS is not competitively priced, and over 10 pounds expect to pay double what FedEx charges, and that for one-tenth the service. Yes, double. It is a mystery to me that this is not more widely known. Never use USPS.
Most of my vendors use UPS, probably because it has more visibility than FedEx. And while I am sure the differences are largely regional, that is, you can have bad experiences with both depending on your area of the country, here in the west I have had pretty lousy service from UPS. The help is consistently indifferent, at times even surly. And their service, while priced very close to FedEx, just isn't as good. And their website! Not the easiest to use, by far. I rarely use UPS. I don't know whether it's a union thing or what, but they're out of the picture for me.
I definately favor FedEx. I have observed that the media has abused FedEx lately. I am surprised. They have been very good to me as a businessman who relies heavily on shipping. Once they spent a week tracking down an errant package, updating me every day, and ultimately offering me a number of options to make it easier for me to make my customer happy. Their pricing is good, their people are upbeat and congenial, and their website is easy to use. They're winners in my book. I use them constantly.
DHL is pretty unique and stands out in one way: this service is very safe, very efficient and very fast. It is also fairly pricey. However, given their special international niche, probably a good value. But using their service is to me very clunky. The website is the worst of any vendor I have ever used, and that is saying something. Because of that and the price, from occasionally using DHL I have switched over to using FedEx International when I have to ship outside the US. I already enjoy good domestic service from them, and their international rates are lower than DHL's. Fortunately, since I no longer do carburetors for overseas customers, I ship only booklets internationally now, and today international issues are few. I don't have time for nonsense.
Tips on shipping
Continuing in their particular brand of weirdness, if you insure a USPS package heavily, it seems to come under more scrutiny and is likely to be held up longer. I once insured some carburetors for more than $1200 and in addition to tying up the shipment for weeks, USPs insisted I fill out papers permitting me to import(?) scientific (?) instruments! Sadly, par for the course for USPS. As for the privates, I don't know how UPS works, but while both offer virtually unlimited insurance starting at $100 free, FedEx has a limit of $499 over which they reserve the right to open the package (presumably to ensure it is packed properly), and the shipment goes out signature-required as well. Signature-required is not a good situation for a business operated by a sole proprietor. I discourage it.
Virtually all the carbs I receive are inadequately packed. A few arrive miraculously, poking out of the box. Pack carefully, assuming the worst possible handling. Bubble wrap is best, surrounded by heavy crumpled paper. Allow a minimum of two inches and preferably three around all sides of the carburetor set. Do not use wood or metal boxes. They do not make up for poor packing. Foam rubber and styrofoam are deceptively incorrect packing for heavy objects. Use more than one piece of packing tape. Line the box with pieces of cardboard for reinforcement. See my shipping tips page for more.