Customers don't design the bags. They pick an available design, then choose colors and leather types, linings, & hardware finishes. They custom-order from the Designer.
If you are thinking of a large group of people making suggestions to 'help' design a composite bag, then buying in, I'd call that something else entirely.
Rarely, a designer will create a unique bag for a specific person with that person's specifications in mind. This would be OOAK in style PLUS the above options. This is considered custom made but is again, not designed by the customer, although her style would never be offered to anyone else.
Once a person has placed a custom order, the leather is cut. No going back. Your designer would be out considerable time, effort and resources if the order were to be cancelled. There would be an 'orphan' bag.
To maximize production for a particular effort, some designers will make a series of XX amount of the same bag ~ enough to justify expense of entire hides in a particular color, etc. Alternately, with others, certain colors will be available only as long as those hides are, but they can offer several styles til it's used up.
So you see the different ways it can work. The Positive is that no matter what, your designer will make sure you are happy with your purchase.
You have a right to your exact leather choices (no substitutions), straight seams and tight stitches. You will have had an opportunity to choose from actual samples of your leather/color choices. If you are confident with your decisions, you should be pleased.
If you are considering new color combinations, get out your crayons !
People enjoy having something that is 'just' for them. We all have leather preferences and wildly divergent favored color schemes. It's fun to put that together instead of searching for your dream bag. It's minimally risky, given the abundance of references, but you have to have faith in your imagination, & know exactly what size and type bag is best for yourself.
If you have to see and hold something to be certain, your expectations may be hard to meet. To help you choose, your designer will suggest certain leathers, because we know which work best with different type bags. Do not settle for a style that's not YOU. If you need outside pockets, you won't be happy without them. If you like a satchel but want a shoulder strap sometimes, make sure that's included. (measure for it!)
The amount of structure (or lack of) is also personal.
*Here's one more tip to assure satisfaction. Do your math. You may use a current handbag, or even a paper bag. Put all your normal 'stuff' in it. Now measure up, across and sideways. Multiple H X L X D. That's your number. It is the minimum volume you require, as expressed in cubic inches ~ no matter what shape bag you choose. (If you like flat constructed bags, it's another formula)
I hope my synopsis helps you consider all aspects involved. Get the return policy clearly understood up front and be assured that a reputable designer is only happy when the customer is.