I chatted with someone about this on the Facebook group. What would the benefit be to you of having a metal shaker? My problem is that most metal shakers wouldn’t be recyclable, so their end of life is identical.
Take something like a Chilly’s bottle, it is mostly metal, but it’s composite, made of a few materials. It’s end of life is landfill, even though it’s metal, because the binding of other materials to it means it can’t be recycled.
But that’s ok, because they’re built for longevity and to reduce the amount of single-use plastic bottles you use. They are designed with sustainability in mind, like the new Huel Shaker.
A 100% metal shaker likely wouldn’t work, it would still have elements of plastics involved/moulded to it so that e.g. the cap closed effectively, or the seal worked.
Also, totally appreciate you having loads of shakers! No reason to get yourself this new one just because it’s new, absolutely get that! We’re replacing our free shaker for first customers primarily 
Are you using a discount code to add the shaker? Or are you just adding the 2 for £10 deal to your basket? If it’s the latter, then it won’t affect your other discount.