1. Go to Support > Addons and click the add-on card (or its Get Started / View Plans button).
2. In the window that opens, read the description. If the add-on has Tiers, click Select on the tier you want (the chosen tier is outlined in green and its button reads Selected). If it lets you choose a quantity, use the - / + buttons under Choose Quantity (limited to the add-on's minimum and maximum). Otherwise the fixed Price is shown.
3. Click Buy and confirm "Your card on file will be charged, are you sure you want to proceed?".
4. Sprwt charges your default card through Sprwt's billing service - as a recurring subscription item for monthly/yearly add-ons, or a one-time charge for one-off items - and shows the add-on's success message. Some add-ons also run an activation step automatically.
5. The add-on now appears in Settings > Billing under Addons and in the Plans table.

You can deep-link to an add-on with /admin/addons?aid=<id>, which opens its window directly.