Here's a fairly typical scenario: I've just sold something, weight approx 1700g, so I selected the postal service accordingly. With packaging, it's now around 2100g. In the past, being cautious, I would have chosen a more expensive service (Royal Mail medium parcel as opposed to small parcel) but now I can't do that, without cancelling the order and asking the buyer to re-purchase. This is crazy. If I were the buyer, I just wouldn't bother re-purchasing. On this occasion, I've chosen just to send the item and hope that it gets through. If it doesn't, the buyer will be met with punitive additional charges. What I will do then is refund the whole amount and tell them to keep the item. I realise some may think that's extreme, but I really don't want my buyers to be put to inconvenience: paying extra money and possibly  having to collect the item from a sorting office.

When you use Simple Delivery, there seems to be an option to give feedback, which I've done twice now. I've given one star on both occasions and left a comment outlining in detail my dissatisfaction with the process. I encourage other sellers to do the same.