One of my biggest peeves about USPS is THEY provide the triangle boxes and then after you gotten used to using them, they have now added a surcharge on anything "over" 30 Inch long. SO....as long as the item being shipped will fit in a 29.5 long cut down round tube that slides inside a 29 and 15/16 long cut down triangle box, you can beat the system on that extra charge.
I remember reading on here that one guy's triangle box was 30 inch long and had a "discussion" with the PO where he was dropping off about their rules saying it was anything OVER 30 inches long. Just keep it under 30 and there shouldn't be a need for any discussion OR extra added surcharge.