Can you combined Royal Mail prepaid labels + stamps for postage?

I wondered if the postage would be valid i purchased a 100g large letter label from Click and Drop and then attached stamps to make it up to the value of a 250g large letter? Or even up to a small parcel?

The reason why i would want to do this is because I have a lot of stamps and used to drop off my mail pre paid for with stamps to my local Post Office, but this recently closed so I now have quite a trek to get to the nearest PO. Yet there is a convenience store just at the end of my street which I can drop RM packages off at, but only if they have a barcode/QR code that the shop can scan.
So if I can buy a lower priced prepaid RM label from click and drop and attach stamps to get it up to the correct postage value, I can get the convenience of dropping them off locally plus also use up the stamps.

Does anyone know if this is going to work or will the packaged get marked as insufficient postage paid, even though it will have enough postage on it with the click and drop label plus the stamps?

FYI i know i can just drop stamped mail straight into a post box but then i get no proof of posting, plus none of the post boxes in my area are big enough to take even the maximum size of large letter, never mind a parcel.