Maybe framing your argument in terms of the direct value (or loss) to their business would have helped.
Maybe something along the lines of this:
"Of course, this is entirely your call. I just want to make sure that we're all on the same page about why I disagree with adding a separate pulldown.
By adding a separate pulldown, you're asking the user to make a choice. One of the choices they can make is to not complete their purchase.
Would you teach your retail cashiers to ask each customer if they were really sure that they wanted to buy today? Are they certain that today is the day to give your business money?
Because that's what adding an extra step to the checkout does. The only possible effect will be to decrease the number of customers that will complete a checkout.
I think that would be a terrible response. Without seeing test rests I'm not even sure no drop down is better considering it's entirely expected by users. Does something that insignificant really warrant such a snarky response? In fact the best stores that I go to always ask things like "did you find everything you were looking for?" before checking out.
no retail cashiers, but point taken, and that sort of was the point - forcing people to make more choices - to think about stuff - opens up the chance for them to choose to not buy, and/or make mistakes.
The user chose to be at this site, chose to pick an item to purchase and then chose to go into payment processing. The idea that suddenly an extra drop-down is going to outrage someone into questioning their entire choice-path is hugely unlikely, and infact the example here is a site that knows it's demographic won't be scared off by what seems at first glance to be a fairly ambiguous user input. If say Amazon tried this I would suggest their A/Bs would have this style of form failing for most demographics.
It's not a matter of outrage, it's a matter of momentum.
Every pause in the purchase process slows down the cognitive momentum of the user, which is why adding, say, forced registration before a checkout is a bad idea.
Admittedly, having to select your credit card from a drop-down is only a small speed bump, but why introduce it at all when there's no benefit and a very real downside?
I would argue there is benefit (what cards are accepted) and little downside. Without testing, I would not be prepared to make a definitive statement though.
Maybe something along the lines of this:
"Of course, this is entirely your call. I just want to make sure that we're all on the same page about why I disagree with adding a separate pulldown.
By adding a separate pulldown, you're asking the user to make a choice. One of the choices they can make is to not complete their purchase.
Would you teach your retail cashiers to ask each customer if they were really sure that they wanted to buy today? Are they certain that today is the day to give your business money?
Because that's what adding an extra step to the checkout does. The only possible effect will be to decrease the number of customers that will complete a checkout.
That's why I'm against adding that pulldown."