Steve says to Jon, 'I am thinking of a polynomial whose roots are all positive integers. The polynomial has the form $$P(x) = 2x^3-2ax^2+(a^2-81)x-c$$

for some positive integers $a$ and $c$. Can you tell me the values of $a$ and $c$?' After some calculations, Jon says, 'There is more than one such polynomial.' Steve says, 'You're right. Here is the value of $a$.' He writes down a positive integer and asks, 'Can you tell me the value of $c$?' Jon says, 'There are still two possible values of $c$.' Find the sum of the two possible values of $c$.