I bought some poste frames that are a bit too small to fit the posters

Honestly, it all depends on how good the posters are. If they’re good or old, you shouldn’t trim them. Go back to the store and explain, or if not, wait til you have the right sized poster. A new poster (not collectible) can be cut down to fit.

Trim the poster to fit the frame.

Absolutely, Alice Jena. Jcraw is correct. If you trim the poster, measure twice and cut once. Poster frames literally do not leave much room for error. ☺️

Hopefully you still have the receipt and can take back and exchange for the correct size. Now you know to measure before purchasing. I keep all my receipts in a file by month. If it is a receipt that will be used at tax time it goes in a separate file.

Ready made poster frames? I backed up a frame with old wallpaper snd centered poster so that it ended up with a small border, much like a mat.