I have a credit but every time I order a gig, it posts from paypal, not the credit in my account.
To spend Fiverr credit the cost of the gig must be equal to or less than the amount of credit you have. If above your credit, it will be drawn from Paypal.
Ask your seller for 2 custom orders – one of the exact credit amount and one of the rest for the gig total. Or ask them to send a custom order for the exact credit amount, and then purchase the rest of the gig via a custom gig extra.