How to purchase add-ons
Click your name in the upper right hand corner of your screen, then click "Billing" to navigate to the billing portal.
You can access the billing portal via the left-hand navigation menu.
Next, click "Purchase Subscription" for Trial users, or "Update Subscription" for Paid users.
Under the "Your Plan" heading, you'll see a section listing keywords, campaigns, and site audit pages. The numbers listed are the total that you currently have available to you.
To purchase additional add ons, hit the "+" (plus) symbol to the right of the current number. To remove previously purchased add ons, hit the "-" (minus) symbol to the left of the current number.
When you hit the "+" symbol to purchase additional add ons, the charges due will display in your "Summary" section to the right. Add ons are now billed along with your main membership subscription. As a result, whenever you first purchase add ons, you'll actually be billed a pro-rated amount depending on how many days are left in your current monthly or yearly subscription.