Some Ideas

When you create a recurring item (sales invoice, payslip etc), when the date come you can see a notice in the respective menu informed you that there is something you have to create, then choosing this menu there is a yellow banner inform you that there is a least one, lets say sale invoice, you have to create. Choosing this yellow banner you can see all the invoices that should be created, but you cannot choose which one to create, although there is a check box, so you have to create all of them. Is that possible to change and choose which one to create?

Second request: Is is possible to be able in billable expense list to show in which invoice the specific expense was charged?

Third request: Is it possible to choose more that one billable expense and charge it to the specific sale invoice at once?

Click Edit for the item in the Billable Expenses tab.

You need to select all billable expenses and billable time that will be on an invoice before creating the invoice from the Customers tab. You cannot add them later.

But this takes too much time since you have to check each billable expense one by one

You can write an invoice with a total amount for the billable expenses and then you can go to the billable expenses list and assign specific billable expenses to this invoice

That is not what you asked about.

This is exactly what I requested

Sorry, @pandhm, but in your initial post you asked:

I answered that you needed to select all billable expenses and time before creating the invoice. You then disputed my comment, saying you could enter a total amount and later assign billable expenses to an invoice. That is a completely different thing, and it does not place the actual billable items on the sales invoice. Therefore, I remarked that your statement was not what you had asked about.

No matter how you interpret my statements or yours, you cannot choose more than one billable expense at a time and assign it to an existing invoice. You can choose as many as you want before creating the invoice, or you can assign them in the way you describe, but only one at a time.

but why?

I believe they should be treated like invoices when you select a customer account, a new field appears so you select inovices, similarly when you select billable expenses as an account a field for expense selection should appear.

Because the selection occurs before the sales invoice is created from the Customers tab.