Recently I have had a number of people come to me with 3D items to be printed. About half of these people were not supplied adequate models for printing and in my estimation were taken advantage of by the sellers providing substandard product.
My warning to you, as a buyer, is simply this…
If you are having 3D models created with the intention of having them 3D printed, please use a supplier that actually does 3D printing so they understand the nuances of 3D printing and can create proper models for the purpose.
Nothing bugs me more than having to charge people to modify a design they already paid for.
Good luck with all your projects!