Though WHMCS doesn't currently have this functionality built in, it would be possible to achieve this using hooks. Alternatively, you could simply give your clients more time to complete payment by extending the Order Days Grace option, and then instruct them to only pay once you have sent them these materials.
Should you want to go down the hooks route, you would essentially need to stop an invoice being generated for new orders of a particular product, and then generate the invoice once you accept the order (and link it back to the order). Another user suggested a similar solution for another community member a few years ago.
You might want to check out our Service Offers and Requests forum, where somebody may be able to develop this functionality for you. You may also want to open a feature request.
Hope this helps a little!