Never an air weapon but I have had two rifle barrels; carriage was reasonable, I seem to remember getting hit for VAT but not import duty. I'd have to try & track down my invoices to verify actual costs but I wouldn't let that put you off buying from there.
As long as you're legal should be no/few issues.
Should you actually need an import licence (don't think you do with EU), they are normally not a problem either.