If you want reliable service, it will take some effort on your behalf.
I use ALS for UOAs at +/-$19 each. At that price, they don't provide return shipping... nor would I want them to.
I take my sample, and put it in a white and light blue plastic Amazon padded mailing bag, and tape it shut. All the postal Service will see is that Amazon bag, and they wouldn't dare to slow it down. Through PayPal, I have an account with ShipStation. I can create a first class mailing label, with a tracking number, for $3.58. I pay for it via PayPal and print it on my printer.
Since it is a pre-paid package, I mail it from my mailbox at the end of my driveway.
It is delivered to ALS in three business days, and I can track it anytime along the way.