No, you cannot group several boxes into a single package, as you would run the risk of partial loss.
No transport company allows the shipment of several packages strapped or sealed between each other, because they cannot guarantee the safety of the content.
Please note that shipments must be labelled in order to be transported. If you purchase the shipment of one package, there will be only one label and the collection note will only mention a single package. As a result, there is no way of proving that several packages were collected. If any parts are lost or damaged, no claim will be accepted.
If you are going to send several boxes, it is best to contract the shipment for several packages.