I am creating a website on which I want to place a form with items for sale. Is there a free way for the form to calculate the completed data and pass it to the payment gateway in a new window?
The second task I need to help is to make it possible to insert a calendar in the form where the customer marks the day of booking? Is it possible to somehow set the availability of only some days and make those remaining days inactive?
Thank you in advance for your time.
Best regards Izzy 
Lots of questions to ask really. Like are the days / hours restrictions on the calendar fixed. Eg like every Saturday no delivery or will it need to by synced with a calendar? And also what payment gateways. Are there multiples and how are you handling taxes. Are there regional restrictions / requirements.
Basically you are wanting a lot of complexity for free.
Sounds like you need to spend some $$ and hire a developer.
You can look at this software : Web Form Builder
Of course, not free, but very good software for making all types of forms, including payment forms.
Hello
Your needs are similar to those of my guest house. If so, there are ready-made solutions for that.
I can give you information if you need it…
I like the scripts of phpjabbers. Easy to integrate into a (Blocs) website.