お席をご利用のお客様はドリンクのご注文をお願いしますって英語でなんて言うの？

The first one is good for a sign. If you say it directly to a customer, it might be a bit strong.

The second and third ones can be said directly to customers, but the third one is the most polite.

Tables or Seats are for customers only this tells them that they must buy something in order to use the seats/tables.

Please place your order/ buy something before taking a seat" politely asks the customer to order/buy something before sitting down.

1. "I'm sorry to disturb, but we ask that all seated guests order at least one item from our menu."
   "I'm sorry to disturb" is an optional polite formality you may use. I'd want to use this phrase especially
   after I approach the customer. However, if you wish to remain direct, you can omit this and just say "We ask that all seated guests order at least one item from our menu."

I referred to the customers as "guests" because I would use this term often when I used to work in the hospitality industry! In my opinion, I think it shows a more welcoming and friendly image, but it is also suitable to use "customers" if you'd prefer!

To "ask" guests to do something is another polite way of requesting a customer to comply with a company/business policy. You can also replace the word "ask" with "request":

"I'm sorry to disturb, but we request that all seated guests order at least one item from our menu."

To ask the customer to "order at least one item from our menu" implies that the customer does not have to spend very much to be seated in your bakery. Your request is very reasonable!

2. "I apologize for the inconvenience, but we request that seats are for paying guests only."
   Similar to the first phrase, "I apologize for the inconvenience" is optional. Otherwise, you can simply say "We request that seats are for paying guests only."

**Additional suggestion: After saying the first or second phrase, I would immediately follow up with "Are you interested in making an order?" or "Are you interested in ordering?" This a polite way of getting a quick answer from the guest, so you can know right away if the guest would like to stay or leave your bakery.

For example:
1. I'm sorry to disturb, but we ask that all seated guests order at least one item from our menu. Are you interested in ordering?
2. I apologize for the inconvenience, but we request that seats are for paying guests only. Are you interested in making an order?**

Good luck!!

Yes, it must be frustrating for owners of cafes when their cafe is full of people all using the internet and nobody is buying anything. Meanwhile, new customers entering the cafe see that it is full and go elsewhere for their coffee. Sometimes there are polite notices on the wall to remind people that they should buy something before taking a seat. A forceful message would be:
"This isn't a waiting room!" although it is not polite. However it does express that the owner is very unhappy with this situation..
"Sorry, these seats are for paying customers only," is more polite.

When in a restaurant/cafe then they often use the term 'customers only' or 'customers use only' when talking about the seats/tables or toilets this means that you have to buy something to sit on the seats/to use the toilets

With problems like this, it is best to keep it simple. Using words like "must" makes it known to English speakers how serious it is and conveys the message you are trying to share.

This is a way you can inform the customers about this arrangement. Reserved means set aside. Therefore it means these chairs have been put aside for people that has purchased something in the shop. Purchased also means bought.