The Shopkeeper Goods Bag is a very large Backpack with a weight of 100 kg. As it weighs a lot, it can be used to help encumber your character to Strength train but any fights will be made very difficult. Although it reduces your Dodge skill effect to 0.00x it is not ideal for training it as encumbrance reduces the dodge XP you gain. (50% Encumbrance would mean half dodge XP)
They cannot be bought, so to obtain one they must be taken from a Shopkeepers. Usually carried by ones in General stores. Visual wise it can be almost impossible to tell them apart so an easy way to tell if a Shopkeeper is wearing a Shopkeepers good bag and not a Traders Backpack Large is to check their movement speed. If they are slowed down via encumbrance they have a Shopkeepers goods bag. ru:Сумка с добром лавочника