Hi cdrhall ,
Thank you very much for your post. I am glad you think the addon does what you need and by your description it probably does.
If I understand correctly, you are looking for a one time purchase. You can purchase a 3 month licence and cancel the subscription. Licence will stay active and last you those 3 months, but the software will not expire. Active licence allows you to keep the software up to date. Remember, that WooCommerce as it is or any plugins authors make changes and improvements. Our developers adapt ATUM and it’s addons accordingly. If it happens that you update WooCommerce or other plugin the addon may fail to work. However, if you do not update anything it should work fine till the end of days.
Hope this helps.