We've all seen on seller's feedback profiles adverse comments about how much actual postage was in relation to the charge in the auction but in essence there really is no need to any buyer to complain as long as they know the cost of shipping up front.
While it is nice for sellers to combine shipping costs on multiple sales it is not mandatory and in some cases not even feasable with is being cheaper to post things separately than to send an oversize or overweight parcel, especially when shipping abroad. I tend to stick to a standard formula where the most expensive charge stands and any others are discounted by 50% but then most of my stuff is sent by courier which enables me to combine a lot of weight easily and offer such a high discount. The postal charges for small items sadly do not really make this as easy when sending smaller postable items.
Remember too that shipping/postage costs are not just related to the price of the stamps. Packing materials cost money too (usually), even second hand padded bags have cost the sender money as they paid someone else for them! Also bear in mind that sellers can live many miles from their nearest post office and they may factor in the cost of fuel or a bus trip to post the items they are sending you.
If a seller is VAT registered, bear in mind that there is no VAT they can claim back on postage costs so if they charge £5 then 74 pence of that is money they have to pay to the Customs and Excise as VAT leaving them with £4.26 to actually spend on postage and packing your items.
Reasonable shipping and handling costs are pretty much irrelevent as long as they are known by the bidder before placing the bid. As it is known up front how much it will be you simply deduct that from the total you are willing to pay and bid accordingly. For instance if you want to spend £25 on an item where shipping is quoted at £12, just bid no more than £13 (£25 - £12 = £13). That way you will never pay more than you intend regardless of how much the shipping & packing costs are.
If the auctions do not advertise a cost then use the "ask seller a question" option to find out the cost. If the seller does not reply or does not give a price then move on to another auction. Sadly some sellers 'decide' how much they are going to charge for S&H based on the end price of the auction to make sure they end up getting the price they want for their item so if you don't know how much you can afford to bid then do not bid at all.
My best advice is to find out and factor in the cost of S&H before you place your bids. Ignore the value of the postage on the envelopes, if you worked it all out before bidding you probably got the item for less overall than you wanted to pay anyway.