Yep, shoulder pain is often the result of tight pecs, but that probably is not the only issue.
First off, you gotta suck it up (financially) and get some ART done on your shoulders and pecs. Chances are that your rotator cuffs are beat up which is causing you to use a compromised position when doing overhead work or chest work (or any shoulder work for that matter). Once they are freed up you have a better chance of letting your pecs get back to where they should be.
Stretches: lots of simple stuff. Lie on your back on a flat bench with some light dumbbells (don't be a dumbarase meathead and think that you gotta get all alpha male on dat chit and use max weight, 2-5kg dumbbells are more than enough), extend your arms vertically then slowly let them drop down into an iron cross position. When you feel the stretch in your pecs, have a little fiddle with the dumbbell position, maybe move your hands up a touch and/or rotate the dumbbells slightly, until you feel you are in the best stretched position. Now stay there for a minute. Suck it up.
You can also stand in a power rack, or even a doorway, with your hands out and down around hip height or a bit above that. Take your bodyweight on your wrists by leaning forward, chest up. Hold it for a minute.
Find a hard corner somewhere, like your power rack upright. Lean into it with your arms crossed so the corner is digging into the back of your shoulder. Roll around a bit and find the sore spots in your rotator cuff. Press in and hold.
No, you don't need to stretch before you bench. But you sure as hell need to stretch after you bench.