The items you get to accompany the first drink of the day are all functionally equivalent to other healing items. Sunday's biscuit is a Sweet Heart, equivalent to a Potion (which costs P200 in these games), while any other day of the week you get an item that's equivalent to Full Heal (which costs P400). By getting one from the cafe, you pay only P198, so that you can get a limited quantity, one per day, at a discount (though it's a very small discount in the Sweet Heart's case). Whatever value you might get from the Poke Beans is then simply a bonus; you can proceed to feed them to your Pokemon and after doing so enough times, the barista will receive word of this and give you free Rare Candies at certain thresholds (a maximum of ten times per save file, and the last of these requires a lifetime total of 100,000 Poke Beans fed to any combination of Pokemon).
If you want to buy more than one drink from the cafe in a day, you can, but after the first one you're just wasting your money unless you really want to hear the barista go over the background story they tell along with the drink. You don't get anything in your item bag or otherwise as a result.
