Just to add some clarity to this thread, the various free offers you may see on Fiverr aren’t all the same, so you have to consider each one on it’s own merits. Some are hyperlinked to specific gigs and the credit applies to those gigs. Other ones apply to everything. On all credits, you cannot buy a gig that costs more than the credit and pay the balance. You have to either get a custom quote to split it or buy a less expensive gig.
If you aren’t sure, take care to read everything you see when you click on a promotional offer. If you still aren’t sure, go to Customer Support and show them the exact link. What is on the forum may not apply to another offer that comes later on, so this issue is best addressed through CS.