Before TradeGecko staff jump in to let you know, you can select ledger account default selection in the settings to default to the variant, the company or the sales channel.
By specifying the default ledger to be specified by the product (let's say a sales ledger), then setting every company to default to a different ledger (let's say a shipping ledger). In theory, any specific product will go to the sales ledger, and any custom attributes added to the sales order, will fall back to the company default which will be a shipping ledger.
This is what I am talking about when I say "a difficult workaround" above. For us, we'd have to manually change nearly a thousand contacts to default to a different ledger. Which is too much work for something that is still a workaround at the end of the day.