Cost of vivera retainer only for upper mouth ?

i use vivera retainers only in upper mouth. But when my dentist ordered them, they got delivered as a set with a retainer for the lover mouth also, but not mine offcourse as i newer got my lower teeth fixed. So i must have been some kind af mistake. My question is when i only use 1 retainer and not 2 as in a set ! How are they ordered/delivered from the manufactor ? Will i get 4 only for my upper mouth or only 2 as if ordered 2 sets (2upper+2lower) And what shoud i pay when its only upper i use ?

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