What's "replace-by-fee"? Is that advice good or bad?

It's a feature that allows you to bump up the fee if your transaction isn't getting verified.

They want to stop you from double spending which you can do regardless of whether you have RBF on or not. It's them trying to paper over a horrifically bad approach they've taken.

In case unclear: This is BitPay trying to protect itself from double spends in the most naive and broken manner possible.

Aren't you the one advocating that mempool policy works? They wrote this before rbf was the default and having it off would actually make nodes reject double spends from the mempool. I don't remember if it was them or bitrefill but they said they knew the risks and saw it as a manageable risk that they could take in order to provide a far faster checkout for some users. As far as I know, they no longer do zero conf since the RBF update but this page probably never got touched.

Filters work for reducing spam, they don't work for preventing double spends obviously.

Is it not the same exact process for both (pre RBF)? Both reject transactions from the mempool but you can bypass it if you bribe a miner to include it. Amex does the same thing. When you make a cc payment from the app, it frees up the balance immediately, 24/7 even though it might leave your bank account the next day. You might double spend them by moving the money out before that, but they take the risk because the convenience drives more usage and they deem it worth it. I personally almost exclusively use it for my business for this reason. I can have full balances on all my cards and pay it off in the checkout line to spend right away. It was probably the same story for bitrefill customers who would buy the gift card they need while in line at the store. Its a significant sales booster and they looked at the risk reward data for it and decided it was worth it for small amounts. It's not because they are stupid, it's a tradeoff that made sense for them. Of course this is before they did rbf everything. No one accepts zero conf now. Heres the video I'm getting this from: