I assume that you're referring to postal imports, rathing than bringing in goods personally. (If you're travelling back from Peru, you're allowed to bring in �145 worth of gifts, souvenirs and other goods, plus your duty free allowance of toacco and alcohol, without incurring any charges).
The rules are complicated but 'quantity' and 'weight' don't normally come into it. The simplest rule is that goods worth under �22 won't attract charges, goods that are worth more than �22 will. (The only exception is goods upon which excise duty is charged. Basically, that means alcohol and tobacco. You'll always get charged for such goods). �22 currently equals �17.67.
OK, simple answer out of the way. Here's the complicated one:
If you import a package from Peru, it will be inspected by HMRC on arrival. (Actually, routine inspection is carried out by ParcelForce International, who've got a contract with HMRC). The first thing they consider is 'customs duty' (also known as 'import tax'). The rate at which this is calculated varies greatly. For example, a basic MP3 player only attracts a rate of 2% but an MP3 player with a built-in radio is charged at 10%. If the figure they calculate comes to more than the current sterling equivalent of �10 (�8.03) you have to pay. Otherwise you don't.
Then VAT is calculated. This is charged on the value of the goods, on the carriage charges and on the customs duty. (Yes, that's a tax on a tax!). The rate is normally 17.5%. However, VAT is waived if the value of the goods is less than the sterling equivalent of �22.
Lastly, there's a customs inspection fee of �8. If there's no customs duty or VAT to be paid, this is waived. Otherwise it's added to the charge that the postman will make before he'll hand over the package.
Chris