Send large letter to Japan with extra £330 insurance

[rassport]

What's your recommendation to send a large-letter sized package to Japan, with insurance to cover the value of £330? The item is a rare music / band poster folded flat to A4. Weight is 200g. I'm really struggling to find an online price-comparer which actually takes into account the higher value. My post office says £250 is the limit and I should accept that, or pay Parcel Force £60. I'm certain there is something out there with insurance for less. Any tips gratefully accepted!

[tobiasd4]

However you send it, you listed as £15 postage to Japan, 

So you can't charge more.

[rassport]

Not to charge the buyer more, but to avoid me paying more.

[tobiasd4]

I'm afraid you will have to do your own research.

Try here, not that I have used them.

https://www.parcel2go.com/