There is nothing wrong with you designing a drawing or writing to be put on a T-shirt, so that if the T-shirt is sold via Amazon, you will get a share of its price, and if it is not sold you will not get anything.

It is not permissible for a partner or proxy to buy something from himself, except with the permission of his partner or the one who appointed him as his proxy. If your partners give you permission to do that, and they are adults of sound mind, then there is nothing wrong with it in that case.

If two people form a partnership, one of them contributing capital and the other contributing labour, then the financial loss is to be borne by the one who put money into it, and the worker loses his labour, unless there was some negligence on the worker’s part, in which case he also incurs financial loss. However, if both of them contribute capital to the project, then losses are to be borne commensurate with the share that each of them put into it.