Job price increase

No you can't as far as I know. You have to be clear with your client from the get go and specific about what you are going to do for them and should there be changes whether you would take care of them or if you would request more money. As far as new services like addition of new colored renders you probably should ask your client to create a new job post with pricing you agree to and then do it that way.

Is paypal an option as payment method? your cgtrader job technically ended so if possible and theres no other options paypals always a good option, and i would ask for like 50% of total price prior to starting work.

I had similar situation once, i asked support what to do and they advised to cancel ongoing job and to create another one with adjusted price. So me and my customer just did that and everything went smoothly. No big deal.