Agree. Even if the code is not applied automatically, at least have a button asking if you would like to use a code in your wallet. Or at the very least have your wallet present and visible in the order page, so you can copy and paste the code from your wallet to add to the code field.
But yeah, Gakken. Many times it's better to wait and shop around before jumping into a PO. Things have changed in the 6 years I've been collecting from Sideshow. POs for exclusives would sell out in minutes sometimes. Most of the times POs for ex's would sell out in a few days to a week, so it was in your best interest to jump on them right away. Now Ex's stick around till right before the items ship, and sometimes beyond. And now the $20 codes have become routine, one a month.
But ultimately it's still a gamble. The 1/6 Ex R2s have all sold out within a day or two of the newest eligible code being available. I'm not familiar with statues and such, so I don't know if those tend to sell out faster.
I don't think Sideshow is intentionally screwing with early purchasers. Some people want to be assured that they get the piece. And believe it or not, some collectors out there don't mind missing out on $20 when they're already spending $500+ on stuff!