We always send the buyer your Common Thank-you Email for every order, regardless of which item(s) they ordered. In addition, we also send a separate, product-specific thank-you email for each digital item they order, and for any tangible item they order that has anything entered in the product's Email Message field.
If you want to disable the product-specific messages and only send a single, Common Thank-you Email for each order, tick the Enable Templated Email checkbox in the settings of each digital product and leave the Email Message field there blank (since we can't generate an email from a template if no template is provided :^). For tangible items, you can just leave the Email Message field blank, whether or not they also have Enable Templated Email checked.
If you were using [%expiry_hours%] in your Common Thank-you Email template, that would not have worked because [%expiry_hours%] is an item-specific variable that only works in product-based thank-you email templates.