P&P charges I give 5 stars if at cost plus a reasonable amount say 30p for clothing etc
4 stars if i feel too much extra added on like a £1 extra
3 stars if not sent by promised method ie paid for 1st signed for and arrived 2nd class
the seller the OP is talking about charging £5.50 for what will cost at most £3.70 (2nd signed for) and and should cost £2.60 as she stated just 2nd class on listing if you change to signed for that should be at your expense not the buyers I would only give her 1 or 2 stars for postage and probably only 2 for communication because she is taking mickey regarding postal charges
secondly regarding knowing postal charges upfront well yes but you assume is someone is charging £3 second class that it is a small parcel costing 2.60 with 40p added for costs and so entirely reasonable however if it arrives with a 69p large letter stamp you have been ripped off, because as a buyer you do not know the exact weight dimension of parcel so you can't know whether it was extortionate,
1 t shirt would have been a large letter so £3 was too much but if she thought well maybe it is thicker than 2.5cm it will be a parcel she was ok with charge but 3 t shirts would still only have been a small parcel so postage should have been the almost the same for 3
if she had sold all 3 separately and sent as small parcels postal profits would be £1.20 ( 3 x 40p) by sending all 3 together and still making a £1.20 profit the correct charge would be £4.80 however as only 1 bag, 1label etc 3x40p is still a bit steep and a more reasonable cost would have been say £3.20-£3,30 instead of £3 by charging £5,50 instead of making 1.20 profit from 3 parcels she is now making £2.90 profit from1 parcel she is ripping OP off still and deserves low stars
she did not say signed for in listing so that can not really be taken into account later as she said specifically that signed for was extra over and above the £3