You have Products by weight and Posters ... somewhere there is a limit of how many Posters fit in a tube and a charge for that tube ...
You need something to "count" the Posters and add the Tube charge to the order ...
Somewhere you need to know that you have UPS and USPS for all Products except the Posters ... and somewhere you need to have a charge for that Tube ...
I don't ship Tubes so I have no idea what kind of charge you want to add when XX number of Posters are in the order and if there needs to be a charged based on a Tube for every XX number of Posters ...
0 weight will give you the lowest price with UPS or USPS to a destination ... but if you want to count how many Posters and charge a particular amount if 1 or more Posters are in the order or if you need to charge a particular amount for every 10 or 20 Posters in the order, you need to explain more what to do with the Poster count ... and then, if the Poster count does become a factor where only 10, 11, 20 etc. fit in a tube then there needs to be a formula for that ... and this is for after identifying how many Posters are in the cart ...
So you need to break this down a little better and help distinguish how do you know a Poster is in the order ... do they all belong to a particular category? manufacturer? anything that marks them as "Poster" that can be tested for? :unsure: