Can I only send messages for particular products?

To send messages only for specific skus you can do this by selecting your Campaign >  Edit Campaign and then clicking on Filter.

In the Filter Options you will see show additional options so click on this and you will see the option Include or exclude products by

Here you can enter the items by inputting the SKUs or ASINs that you want to send the message for.

You can also set your campaign to ignore certain SKUs so messages are not sent for these.