As
@Austeyrhas advised - always assume there will be an import cost
First make a valuation based on the item cost plus the postage cost. This gives you a guide to the valuation, particularly as customs can include postage - this does not always happen, but plan for the worst case and also they may reject the declared/invoice valuation and estimate a market value
Start with VAT - assume you will be charged 20%
But that means there is admin at customs, and the carrier at the port of entry handles that - so you get a handling fee on top. This varies and may be a fixed amount or a percentage of the valuation
It is very unlikely, but overseas retailers can now register with national tax offices to handle domestic VAT. If they did then your checking out on the transaction would include VAT and they detail that on the declaration with their HMRC registration - if this occurred then you would not get a charge on import and therefore no carrier handling fee
However, NI sits in an exclusive position following Brexit. You continue to have a land border with Europe and a complex agreement in place to minimise trade friction across Ireland
If you import from Hong Kong etc then charges will be due, but if you import from an EU airsoft retailer it could get interesting and you could benefit from the Brexit mess