Do you turn on the blades with a switch or a hand lever?

Also, does he work underneath the mowing deck to repair it?

I engage the blade with a hand lever and I really can't remember if he works under the mower or not but I know it dosen't take him but a minute. Guess I'll load it on up and take it in. This time I will watch him close. Thanks.

Ask him for an explanation of what he does. I can't think of anything that would only take a minute.

The nut that holds the blade onto the shaft in the mower deck, that has the large pulley on top, and is turned by the belt, has probably been installed without the lock washer that keeps the nut from turning!
When you take it in this time, ask them to inspect it to see if that lock washer is missing. If it is, and they install a new one, it will cut down on the number of times they can charge you to tighten that bolt, which is why i think they haven't fixed it right. how much do they charge you every time they tighten the nut?
There are two types of lock washers. One has a serrated type of teeth formed in the inner, or outer edge of the washer. The other type is the standard sprung type, as used for many years. The first type is used when there isn't enough length of the bolt to use the other sprung type. Also, you can take the nut to a hardware store, and buy a nut that has a nylon insert in it, which when you tighten the bolt, the nylon insert keeps it from loosening.
Been there, done that! Rustyj