Mass balance is used to show that the input and output values for your LCA model are balanced, and it is required for your Background report.
In order to manufacture the final packaged product, you need to have at least enough raw materials that go into the product and its packaging, and there may also be ancillary materials and waste that need to be accounted for.
Often it can be beneficial to check the mass balance after data collection is done and before you start modelling to ensure there are no gaps or anomalies in the data.
For a simple mass balance of the product stage (A1-A3), the following should be included:
1) Raw materials in
2) Ancillary materials in
3) Packaging materials in
1) Packaged product
2) Waste material
The attached example can be edited to fulfill this purpose for your Background report.