Givi doesn't want the liability. They have no way to know how and by what methods a person will mount their product so of course they will issue a blanket statement recommending against such actions. That doesn't mean other methods won't work.
My 02 Yamaha Vino scooter has a maximum capacity of 165lbs per Yamaha. It states that in the owners manual and on a sticker on the back. I weigh 190 lbs. and have ridden it for almost 8000 miles. It wheelies surprisingly well and has hauled up to 5 people (at the same time!) around the pits of many racetracks in the US with no ill effects.
The rear rack on my BMW has a max capacity of 10 kilos or some such low number. I routinely put twice that on it.
I can produce many more examples of my stupidity/luck but back to the GIVI on the SD.
I can grab the thing and torque it in all directions and it is secure. Sure it flexes and gives a bit but again, I have zero worries about tossing a weeks worth of stuff in there and heading out. Shoot, what are we looking at, 25lbs? The seat flexes and twists a bit with a 150lb. passenger. (yeah yeah yeah, some of their weight is on the footpegs etc. etc. but the majority is on on their behinds) The only real difference with the case would be the load under braking and again, the weight is overall so small that I can't see it being a problem. The only possible failure I can imagine is structure weakening of the plastic seat base over time due to the repeated torsional loading. I will keep an eye on the seat base I suppose.
I promise I will report back with any failures. (and eat my words if I have to!)
BK
not afraid