I have a generic line that I want to show in the short description of each product and at checkout (i.e. buy one and get one free). Is there a way to automate this rather than for me to enter in to each product?
There are multiple ways to do this. You could add the text in the phtml file and maybe add some variables here and there. That way, all products have an identical description. You could also do a check here: If a product has a value for the description, you could show that description. If not, show the default description. In that case you'll make the description attribute optional, so you'll need to change the required setting for the attribute.
Another option could be to use a custom variable here. That way, you can just add the custom variable in the product description and it will show the same text. Downside is that using product specific data, like product name, can't be shown in the description.
Final option might be to make this configurable. Create a custom extension where you make it possible to set the default description in the system configuration. You could develop this in a way where you can use variables within the text. You can show this by default and maybe still make it possible to create a custom description for a product as well, which will be used instead of the default description.
I hope this will help.