Hello,
It if is a hot wheels car still on the card with the blister then it will be to large to go as letter mail and must be sent as a parcel.
If it is a loose hot wheels then depending on the size you may be able to send it letter mail but I would advise against this because you would only have room for the car itself. You cannot use any additional packaging ie wrapping the loose car in bubble wrap first otherwise the parcel would become to big. There are only a handful of cars types where you could do this. A flat indy race car would probably work but again the vast majority of loose hotwheels will need to go parcel as well.
It should be noted that I have seen sellers sell both loose and MOC hot wheels and offer letter mail. I feel like these sellers are rolling the dice. If either piece did get through letter mail you are risking damage to the product espcially the mint on card hot wheels.
Hope this helps.
