In other words, it has nothing really to do with the user being logged in or not, nor does it have anything to do with the destination. The problem is based entirely on multiple items exceeding allowed limits.
There are two ways around this..
1) The BEST method.
Sell these items in bulk packages (apparently that is the way you are expecting people to buy them). So by pre-packaging in lots of 10 you can keep your parcel size minimal.
2) The worst thing you can possibly do if you expect accurate quotes all of the time, as well as running the risk of creating a parcel that is simply too large for any given carrier to handle.
Enable the 'constrain dimensions' option (ozpost v2.0.6).
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Yeah, I know that you, like almost every other ecommerce merchant on the planet are thinking there must be a better way, but the fact is, there isn't. There are some methods that are better than others under certain conditions and there a a lot more sophisticated systems around than ozpost, but it ultimately comes down to having to obey the laws of physics.
If any given customer adds enough items of any given size to their shopping cart sooner or later the contents WILL be too heavy or too large for any of the normal carriers to handle.
Cheers
Rod
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