this is why we try to deal locally even if we need to get special orders.
when you reply to them include detailed information on the modle of the gun including links and copies of web pages of either the retailer or the manufacturer or both that state the fps of the gun.
what is the item in question? if its something way out there and rare it could be just they are unfamiliar with the specific item.
things i can think of.
if the website says 350-400fps then the gun "could" shoot below 366
if the gun has an included "barrel extension" (read supressor) that may be an issue.
if its a tm of any kind itll likely still be on the ban list thing so even if you had it pre upgraded with documentation it could be an issue. (though i see alot of tm retail so who knows the current method of import for those
anyhow it does happen but knowing the item may lend a clue as to why its held up.