Asheirah

Hi everyone, I was wondering if there was a way to calculate shipping based on order size. I mainly sell journals, and if they buy one bulkier journal, it can still be sent as lettermail, but as soon as there's more than one, the package would have to be sent as parcel, because it'd be too big for lettermail. Is there a way around this? I've looked on the forum already, but it's not exactly the issue I'm having. Also, Is there a way to add a decimal in the weight? because these items are very light, as in 67g for a small journal, and like, 2g for a bookmark. The system will only allow me to add non-decimal numbers, so I left it at 0lbs, and the journals at 1lbs, even though it's 'only' 300g. Thanks in advance.