Hmm, I might have to have a play with the postage settings to see if I can work it out, but unfortunately I doubt it - I have postage discounts that work across three services, but it's all extremely simplified - basically I have regular postage, registered postage and express postage, and a flat rate discount of $0 for all additional items, which applies to whichever service the buyer chooses at checkout. There might be a way to do it with promotional postage, but I'm pretty sure that any eBay postage discounts that are set up this way have to work like mine do - that is, if you offer more than one service, it applies to all services.
i.e. I tried to do it a different way recently but found that there are some things that are just impossible - my items generally go as letters, so when AP upped the price and introduced priority, I actually wanted to have a basic postage rate, then 20c per additional item up to a max of $5, but only on standard and registered post, while express remained as a flat rate with $0 per additional item because parcel prices didn't change, and there's only the one weight bracket for up to 500g.... But it can't be done... no matter what, my discounts applied to each and every service offered.
A store promotion may be a better way to go - they get applied to item prices, but same deal in the end - eg $x off for every $x spent, and then you could put in the tag line " + free express post", they'll show up on qualifying listings to advertise the promo, too.
