Hi Laura Curtis,
Customs value of the goods at the time of importation is determined by the value of the goods, postage, packaging, insurance and any duty you owe. You pay VAT when you buy the goods or to the delivery company before you receive them.
If you have to pay VAT to the delivery company, it’s charged on the total package value.
More information on thresholds explained in below link.
Tax and customs for goods sent from abroad
Thank you.