I found one of my sticker sheets (no. 18) in the ad you mentioned:
So until No. 18 they were definitely released in 1998. If the numbers 46 and 48 are also from 1998 I can’t say. So there is still some uncertainty as to the actual release date. EDIT: Buuut! Judging by the looks of the 2000s stickers I posted below, it seems that the ochazuke stickers were a completely different badge. That seems to narrow it down to 1998/1999. What do you think?
Besides that I also found another sheet with Starmie on it – now I just have to find this sticker sheet! 
After searching for: “永谷園 お茶漬け ポケモン” I found those interesting 2000 Nagatanien stickers in the background (which may be very helpful for other members)
The year 1999 did feature one stickers in the ad.
Interestingly it is the same sticker as in the 1998 ad.
Make what you want of this.
Best,
orannge