When RM24 and RM48 methods of postage were introduced they were not covered by any handling agreement between Royal Mail and the Post Office (unlike all other forms of mail, for which RM pays POs a handling fee for each item) and initially POs accepted these 'drop-offs' but received no income.
Last year RM reached a handling agreement with the PO and a 'supplementary charge' was introduced for both RM24 and RM48 items; initially it was 15p but rose to 20p at the beginning of last month (April 2024).
The supplementary charge only applies if you take the item to a PO. Now it may be that you are unable to request collection when purchasing the label, in which case eBay is (correctly) forewarning you that you will be charged an additional 20p when you take the item to a PO. You do not, however, have to do that unless you want proof of postage - you can drop the item in a parcel collection box and that will not incur the 20p surcharge.
By the way, your second paragraph ("Every label I print now, via eBay postage, says a supplemental charge will apply even though I double check I've ticked the 'drop off' box.") is a little confusing. Did you mean you double-check that you've unticked the drop-off box? - because if you ticked the drop-off box then it is correct that a supplementary charge will apply. But only when you actually drop it off at a PO.
