I ordered 6 bottles from Sains with the 25% disco along with my weekly groceries.

Order arrived with only 5 bottles as they were OOS of one of my choices. I didn't notice it at the time but I was charged full whack for the other 5.

Seemed sharp practice to me and, after much whinging via email and phone they sent me 3 extra bottles FOC.

Other suppliers deal with this differently. For instance, 3 for £10 are auto priced at 3.33, 3.33, 3.34 rather than 4 + 4+ 4 - £2

So...one needs to order (say) 8 or 9 botts with Sains in case they are OOS of some.

Discuss

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Four wheels move the body - 2 wheels move the soul.

I can understand that if they only sell 5 then they won't apply the disco. However, I'm Surprised they didn't try to offer a substitute for the wine oos to keep the offer applicable. That said, it's likely never to be a wine you'd chose to purchase.
Like you say, safest bet is to order a few more to guarantee you make the 6 quota. I'd think you should also be able to return the wines as they didn't go for the price when you checked out online.