I reached out to conference for invoice of conference registration. However, I was told that they only issue a receipt. There is not way to issue an invoice of my registration. I was little confused about the response of an international conference. I wonder if this is common to accept as it is or I can further discuss conference organizer about it.
I have organized conferences. The majority of participants will be served well by the conference's standard operating procedure. But there will be a minority which may not be so small, who require special help. For instance: invitation letters, participation certificates, receipts and invoices that need to be formulated in a specific way or need to mention project names or numbers. A good conference organizer will try to accommodate as many of these requests as possible. After all, many of these requests will stem from problems that the participant would otherwise have with getting reimbursed, or attending the conference; nobody benefits from that.
The tricky bit is that many of these requests come in the middle of a very busy time for the conference organizers. They are organizing a conference, after all. Especially for the bigger, international conferences, attendees can number in the thousands, and the number of things to do can explode when the conference comes nearer even without accommodating individual requests. I can imagine that organizers of such conferences decide on a blanket ban on non-standard procedures. It isn't nice, but I can understand it.
Ask yourself: do you really need an invoice and a receipt, or will one of the two do? Are you in a hurry with this, or can it also wait for a week or two (maybe the organizers will have more time then), or even until after the conference? If the invoice and the timing is of critical importance to you, you might consider insisting in a further email. But ask yourself whether it is worth it.