What is the ruling on designing blouses for an online store and putting words on them, while receiving a profit that varies according to the value of the blouse?

The aforementioned transaction between you and the website is permissible, considering it a partnership between you. You participate by designing the form and writing the content, and the website handles manufacturing, marketing, and sales, with you receiving a known share of the sales profit. There is no harm in the profit varying with the selling price, as long as the agreement specifies a known, not an unknown, share of the profit. Indeed, profit in a partnership is based on what the partners agree upon. Ibn Qudamah said: "The profit between them is according to what they have agreed upon in all types of partnerships."