Ok, as stated by @carolhmj decals will work fine. Yet, there’s another solution (there nearly always is in BJS 
. As used in my ‘BSA Bantam motorcyle project’, you can create another mesh for just your personalization portion of the t-shirt with just a tiny-mini little offset from your t-shirt mesh. Sure, it creates (in your case 4) more portions of meshes but the advantages are:
- you can use this to create a bump/embrodery or flockage effect on the design without affecting the t-shirt texture.
- you can easily change this texture or remove it (although you can do the same with a decal).
In the end, decal or offset mesh will provide a similar effect. It’s basically a choice of preference towards the handling. In any case, the only thing I would recommend is not to choose an option that would affect the ‘plain’ look and rendering of your t-shirt. Although, it might feel ‘sexy’ to work it with say a node material, it would be just too much trouble (my opinion only). Let’s just keep things simple (that would be my moto