Most models currently require a subscription in order for fans to purchase store content. This is completely at the discretion of the individual model, so not all store purchases require a subscription, but, currently, most models require a monthly subscription, first.
The good news is that, if you subscribe for a month, then set the subscription to not renew, you can make store purchases during that month you are subscribed and continue to have access to the private media purchased from the store, even after the subscription ends!