How to decline a postdoc offer a few days after accepting it?
Being on the receiving end is very unpleasant, and it will have probably cost them more than just 2-3 days; the bureaucratic arrangement, possibly losing other postdoc candidates, having to go readvertising etc. is a very serious hassle, let's not pretend it isn't. Being there, done that.
Nonetheless, you need to take a decision and inform them as quickly as possible. There is no point for you taking a postdoc that you (and ultimately them) will regret. Decide and inform. This is an unpleasantry that is part of life. Yours and theirs.
To contain the damage as much as possible, you need to face it upfront. Good luck!