Help with condolence card?

My boyfriend's grandfather passed away the other day so I'd like to send a card to my bf's parents to express my condolences and best wishes. I've never met the grandfather in person but since I know my bf's parents pretty well I think it would only be appropriate.

The problem is I have no idea how to write such a card. I've written letters in Japanese before but this is a delicate situation so I don't want to offend anyone. How do you say "my thoughts are with you" or "I am very sorry for your loss" in Japanese? And somehow it feels too short if I only wrote that. But since I've never met the grandfather I don't know what else I could write to make the card more personal.

Could someone please help me with this?

Also, it will probably take a week or so until the card arrives which will be quite some time after the funeral. Is that too late for expressing condolences? I don't want to make them sad all over again when everything is supposed to slowly go back to normal.

Thank you very much in advance!

I've written letters in Japanese before but this is a delicate situation so I don't want to offend anyone.

If that is the case, the most thoughtful thing you can do is to write the card in English, and if you wish to write any Japanese you should ask your boyfriend for advise rather than to ask anonymous posters like me on the internet. And no, a week is not too late for a card.

But a typical condolsence phrase is
お悔やみ垂ｵ上げます。(o-kuyami moushi-agemasu.)

This is just for reference.
http://www.teglet.co.jp/naoko/

Unfortunately his parents don't speak any English so writing the card in English isn't an option.

And of course I've asked my boyfriend but he's not very good with stuff like this. When I asked him what Japanese people usually say in situations like this he said he didn't know. Also, it's his grandfather who passed away and he's mourning as well so I didn't want to bother him with this too much.

The internet might be an anonymous place but I've seen many people on this forum give really helpful advice. And if someone decides to post something which is not helpful I'm free to ignore them right? So I don't see what is wrong with asking here if I don't know who else I could ask.

Thank you for the link, I'll check it out.

If you reside in U.S. find the local Japanese Buddhist Temple and they will help you.

And I hope I have answered all your questions.