Are the buckles sliding off the end of the (too short) straps?

Can you take the helmet to REI and ask them to sew a stop (like a hem) on the end of each strap?

If the length is good for your fit, and it's just that the buckles don't stay sometimes while the helmet is off, maybe a stop at the end of the web would work?

If they come off or come undone while you're wearing the helmet, that's different. I think I'd contact the manufacturer for advice at that point because of the safety issue.