The import itself is rarely the hard part. The hard part is that data kept in a spreadsheet usually has problems that only surface once something else reads it: the same product spelled three ways, barcodes stored as numbers so leading zeros have been eaten, prices with a currency symbol in the cell, two rows that are actually the same item.
So the honest sequence is: send a sample first, we tell you what will break, you clean that, then we import. Skipping the sample step is how people end up importing a mess and then cleaning it twice.
One thing worth deciding before you import rather than after: how product names should read. The interface is translated into every language we ship, but the names you type are not - whatever you import is what a local employee and a customer will see on screen and on the receipt.