Import xslx/csv products with additional_shipping_cost invalid error

[Catim]

Good afternoon, 

We are trying to import an .xslx (or a .csv, it's irrelevant) with 1000 products approximately, but there's constantly the error of invalid additional_shipping_cost. We've almost have tried everything: empty, 1, 2, 3, other kind of values but nothing works.

The mistake that appears is the following:

[product] TINTA BRO LC-123 BLACK 20ML (ID: No ID) it can't be saved
The property ->additional_shipping_cost is invalid

What else could we try? Are we missing something? Any help is welcome. 

Thank you very much.