i think, you need to pay 20 dollar for getting monstrous, and you paid 10 dollar now? i think, you need to pay anyway 20 dollar that would be 30 dollar, you spent. i've told you to pay 20 dollar instead to get access to all files at once.
If i being honest: i don't test this my self, but i think, that, if i pay 10 dollar, and want something, that is 20 dollar, then i need to pay 20 dollar. i am not sure about this.