Putting all 10 codes on each line is the only way to do it using Send Stored Codes. Each line can be up to 255 characters long. It may look jumbled in Seller Admin where you paste in your list, because long lines get soft-wrapped, but rest assured that only a hard return counts as the actual end of line. If you're using a Mac, beware of this known bug with Adobe's Flash plugin for Mac:
That said, if putting all 10 codes per line will not work for you, in that case you may be able to devise a custom solution using our Send Generated Codes feature:
For that, you'd maintain your master list of codes at your end and have a custom script on your server that would output the codes when we contact it. We take whatever raw text output your script outputs and forward that to the buyer as-is, so you can have your script format that output however you wish (e.g. as 10 lines with one code per line).
If you'd need some help setting that up, we can recommend the competent, E-junkie-experienced developers listed in our directory here: