Most packages are insured up to a certain amount as part of the courier service. This means that if your parcel is damaged or lost, the company will offer to cover part of the cost. If you're sending something very valuable you'll probably want to purchase additional cover, so remember to take that extra cost into account.
However very delicate items, such as china or glass-based pieces, will only be delivered via an economy service - which typically won't include any compensation. This is because there's a high risk of the item breaking, so if you want it covered, you'll probably have to go with a special service.
It can be very handy to have a rough estimate of how much a parcel is going to cost, especially if you don't have time to go down to the post office to get an official estimate. This can be particularly useful if you're selling something on a site such as eBay, as you'll need to note how much customers will have to pay for postage and you don't want to underestimate, otherwise you'll lose money. To get the best price, remember to shop around and only pay for extra services if you really need them.