Just say stickers, their brains can process that as the computer doesn't say no, usually though all it boils down to is an extra sticker so that if it's on the dangerous list it's trod on a little less hard by the sorting staff.
I generally always use the shop type ones, i.e not staffed by Royal Mail as they operate with more common sense and will not argue if it doesn't fit their template order (the occasion where one got in a semi screaming match with me then being near mute over the buyer of a Car Radio not wanting insurance was quite memorable) as long as it fits the brackets and isnt listed as prohibited.
I generally always use the shop type ones, i.e not staffed by Royal Mail as they operate with more common sense and will not argue if it doesn't fit their template order (the occasion where one got in a semi screaming match with me then being near mute over the buyer of a Car Radio not wanting insurance was quite memorable) as long as it fits the brackets and isnt listed as prohibited.