You've already checked but be aware that the dimensions could be an issue where the dimensional "weight" for bulky items costs more than the actual weight and has a higher postage cost or could incur a surcharge.
Lastly, invest in a scale and stop overpaying for postage - this is more money in your pocket. Scales are relatively cheap; available on amazon and accurate enough for postal use.
I use this scale and is good for up to 50lbs with an accuracy of .1oz for items less than 20lbs and .2oz for 20-50lbs.