As long as you use the same Apple ID account, you cannot be charged twice for content that you have successfully bought previously.
When you go to the Downloads Page you can click on 'Restore' and this will take you through the process for activating the full plan that you previously purchased. Alternatively when you are asked to purchase a full plan, you can accept and the process will identify that you have previously bought the plan and make it available at no extra cost.
Note that you must use the same Apple ID that you used to make your previous purchase. Perhaps check through your emails to find the original receipt.
We do not have any access to your Credit Card or Account details therefore if you are unsure or have an issue accessing your Apple ID, please go to the Apple ID Support Page here https://appleid.apple.com/