I would suggest you double the price of the plant stock and then use the same labour rate that you have for tree work. That would be your regular charge rate minus equipment costs.
The stumper doesn't create a good planting hole by itself, but it churns up the dirt nicely to make it easier to dig and gets rid of any roots. Stumpers are great for preparing soil trenches for hedges.
I wouldn't charge for root stimulator, just add it into your bid. It isn't expensive and may save you a call back should the tree fail.
Are your soils so variable that you need soil testing? I would suggest you talk to your local extension forester who can probably give you a good handle on the variety of soil types in your area, then develop a planting protocol for each soil type. Also talk to the various nursery reps. So, if you are in an area with eg silty/clay soils, dig all holes larger deeper, amend backfill with sand and topsoil and 2 parts magic pixy dust. The nursery reps, especially the ones from the large wholesale nurseries, as they deal with all the landscapers and find what problems/solutions are around. I think they would be a great resource as it is in their interest for you to succeed.
Which leads to the next issue, guarantees. Are you considering guaranteeing your trees? That could be a good selling feature, but you have issues with ensuring the tree gets sufficient and proper watering. Also check the nursery paperwork. The largest local wholesale nursery voids their tree stock warranty if your remove the burlap and/or wire cage from the trees, even though this goes against industry standards.
I'm not sure what you would be selling with a continuing care package. For the first year, as the tree gets established, all it really needs is proper watering. In the second year, maybe some light structural pruning and a little fertilizer. Would it be better to sell this as a call back the second year, or as a package the first year - don't know the answer. You'll have to figure that out for yourself.
Finally, after you've planted the tree, a doughnut of really nice white chips around the tree gives the HO the impression you've done a really good job.
Congrats on expanding your horizons, I think you will do well. Next you should consider some plant health care, such as fertilizing and pest management. If you want a niche, then consider pest management without chemicals.