Patreon/coin top up

I am reposting my question about the Patreon. It was introduced as a way to use PayPal as a payment method. But Patreon is a monthly subscription- which i don't have a problem with. BUT my question is if you need to top up before the month is over, how does that work? Will I be charged continuously?

For example if I initially joined at a $20 tier but need to add $10 and/or $30, will i get charged for all of those monthly? Separately? Will i get billed next month for the $10, $20 and $30 tier? Or - Do I have to end out on the tier i intend to keep, meaning if I bought all of those, i’d have to spend an additional $20 ( or $10) to be billed the $20 teir i’m comfortable with?