yo and rkuchinsky thank you very much for your input. KenoLeon, no worries, just feel free to ask more as this topic could become a good reference for future relevant inquiries.
yo, the upfront minimum i assume is something like a downpayment right? Let’s say the first couple of months to be payed upfront. Will definitelly ask the lawyer I am collaborating but I wish to hear he tips and tricks from a designer’s point of view and experiences working under such collaboration pattern.
rkuchinsky, lot’s of good info. Equity I agree is not my most preferable way. Royalties as well, you are right, as I will not have total control I cannot predict the sales and as it is a new company I have no idea on the way they plan to sell. Retainer seems to be the most rational approach however I have some concerns.
Some more info on the background story and future collaboration with the client :
The last year I finished the design of 10 products (softgoods, not shoes 
) and I estimated a price per product x 10 (similar products and similar effort). That was a flat fee. The estimation of my salary was done based on the target retail price of each product. Our ongoing collaboration will involve the design of more products but also POS systems and other brand promotion-related objects (bags, tags etc).
a) By billing/hour I will need to either “reveal” how much time I need to design a product (do not want to do so) or I will need to “lie” (let’s say for a product I need X hours but I will claim I worked 2X times in order to justify the real price I value my designs. By “design” I mean the value the product will bring to the client).
b) I understand a retainer (hourly rate) in the “background” projects such as designing POS, shopping bag etc. But I am not a big fan of hourly rate in product design work. Do you think it makes sense an agreement that “background” projects will be tied to a retainer and product-design-development to a per-project basis (as we did until now with a flat fee).
To better illustrate my concerns, if I am asked to design a man’s belt (sketches, 2d designs/renderings). Then, let’s say I will work (approx.) 3 hours research- quick hand-sketches, 0,5 hour discussion, 2 hours for 2D designs to give to client and supplier. Total 5,5 hours X 100 euros (106 dollars) = 550 euros (586 dollars) which is pretty low in terms of what I would consider. For a very similar project I charged 2000 euros (2133 dollars or 387dollars for 5.5 hours of work). Of course If I mention to him that my /hour rate will be 380 dollars and for example for an 1 hour phone call (usual duration of discussions) he will be charged 380 dollars sounds a bit too much… am I right?
Just to let you know, his products will be retailed in prices of minimum 500 euros going up to 2.000 and some products with exotic materials up to 5000 euros.
I am sure Richard has been to much more complex types of collaborations, this for me is the first on of this scale and feel a bit puzzled.
Thanks again!