Ok let me write a post to give a general idea re Horizon/ Speciale Commande. This is gonna be long.
My DH and I were given a presentation on this and subsequently we both had bags made. Our then home store SAs have been trained and are in charge of this aspect now, however we did our orders directly with our Paris SA because we have actually shopped with him the longest.
Generally Horizon covers any item in the Hermes Universe which is custom made or modified for a customer. This can range from yacht/ car interiors, built in wardrobes, furniture to of course bags. As it is a case by case basis, the process is very, very time consuming and requires patience and deep pockets. Included in the Horizon folio is the famous green Apple case which was made on request for a lady as a gift for her gentlemen friend. It holds his favourite snack and a knife.
Also of note, the wheelbarrow which was made for the Duke of Windsor as a gift for his duchess Wallis as the suggestion at Hermes for more gloves and perfume as gifts was met with "She has barrows of gloves". So they made her a barrow FOR her gloves and perfumes.
There are some examples of bags and under SC, one can request for discontinued models to be remade but these are generally not modified and you are definitely not allowed to modify a Birkin or Kelly.
So, when one is invited and expresses interest, the client's requirements are taken down and sent to the design studio in Paris. At this stage only a general ballpark quote is given and no payment is required. I would say an estimate of 3 times the price of an equivalent PO bag is not unheard of.
After a long wait, as is usual with Hermes, one would be given a design presentation in the form of sketches. Modifications may be made at the point and a deposit is taken. After that the final design is presented and for bags these are made up of white leather which gives the client a chance to try out the bag, examine pocket placement, strap length etc. I believe these mock ups are stored in the Hermes archives and every bag they have ever made exists as a white leather 3D mock up. At this stage they will explain that while the design rights belong to Hermes they also offer that no one else would be given the chance to have this bag made unless approved by the client. Now this is not as clearly set in the contract as before so I will leave it here and wait for those to who have experience to contribute. I have to add here that I gave my SA a very hard time and designed my own bag with Hermes elements. My DH was much more accommodating and went the normal route, leaving it to them to fulfil his wishes. Where leather is concerned, if they have it, it theoretically can be made. Custom hardware will of course add much to time and cost.
Again we have a interminable wait and if SOs seem to take ages, this would be more like an eternity. Needless to say final payment is make on collection. I understand that for worldwide SC orders, they are done at the atelier in Pantin. My DH and I were very fortunate to visit the SC atelier upstairs at FSH and meet the craftslady who made our bags. It was a chance for her to see my bag In Action and for us to thank her personally. We also met the other 4/5? craftspeople who now only work on bespoke bags.
For me it was a dream come true, to have a bag designed (by me hahah) and made for me by Hermes. It is one of a kind and my real treasure. I would advise anyone to consider it seriously and also to put in a request for an element which personalizes it. Only you will know it is an Hermes but experienced SAs and craftspeople will recognize that it is from the Maison and will be very intrigued about it.
I guess I need to ask my FSH SA abt that. She would probably think I’m nuts having it made just for my makeup brushes LOL 
