I only want one paid calendar. Do I have to get a full account?

Yes.

There are several reasons for this.

• Most users only have a couple or at most a few subscriptions. You should consider access to all calendars as a discount to heavy users rather than an extra payment to regular users.
• The price would not be significantly lower than the current account. Paypal's share increases as payment sum decreases. If effect, the discount would at most be something like 20%. For the cheapest account, this translates to a discount of just 0.06 USD per month.
• The price is very reasonable to begin with. The cheapest option (5-year account) costs just 0.32 USD per month.
• A single subscription account makes things complex. There must be some way of upgrading the account without losing the remainder of the time he has left. Would multiple instances of the same calendar count (i.e. two locations for the Weather calendar)? Providing support for the added complexity is not worth it.
• It would be easy to circumvent the single calendar limitation by downloading the full content of a calendar, then switching to another one and downloading that. Or should the account be fixed with a single subscription which could not be changed? Just more unnecessary complexity.

Even if you use only a single calendar, is it really not worth even 0.32 USD per month?