Wow asus is one of them, iam a little surprised.
"after 4 weeks they let you know they couldn't read the invoice number or something like that"
Did asus really do that? just want to know.
No, that was just an example. But I do have some experience with Asus promos.
When I tried to redeem an Asus promo code, I had to pick one of their regional redemption sites. I bought an Asus product in a shop in a different country which shipped it to me. When I tried to submit the form for the region of my country, it would not continue to the next page nor show an error. When I emailed customer support they said it could be a browser error. On several website other people had problems redeeming their Asus promo codes, as it appears the redemption sites went offline or 'out of stock' in the weekends.
Eventually I started to use my brain and went to fill in the form at the redemption site for the region of the shop where I bought the product. And that worked.
So I am an idiot. But still, Asus should have displayed helpful errors. They could have been more clear which one of their regional websites to use. Or better yet, have a single universal redemption site.
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Another personal experience is with a different brand, Bosch (they make home appliances and tools). After sending them the invoice, they mailed me they could not read it. They asked me to resubmit the invoice through a different form. Apparently it is not possible to use the original form to resubmit the invoice, it causes a 'serial number not found' error. At first I got a form that did not even contain proper labels, just placeholders. The form itself did not work either. A few weeks later I tried again. The labels were fixed and I tried saving the invoice as JPG, GIF, BMP, PNG and PDF, but the result is always the same: "Failed to upload your invoice". As of today this has not been resolved.