Help at the Post Office, mass letter sending (Madrid, ES)

Hi! I'm looking for a RESPONSIBLE, adult person, who will be able to help me locally in Madrid. 200 EUR for one day of your help. I am managing a project for one of our clients (we are an Advertising Agency). The task is to handle sending a traditional paper mailing campaign (printed, letters in envelopes). I need help from someone local from Madrid to assist me and translate everything at the Post Office, basically get the job done. I will need an invoice (if possible) and i will be paying with credit card (cash is also an option, but i need to know in advance). I will have around 20 000 (twenty thousand) letters to post, each at the rate of normal, untracked letter (what's the price of a standard local Spanish stamp now?). I'm offering 200 EUR for your help, everything shouldn't take more than a couple of hours. I will personally be there with the letters, but i don't speak Spanish, so i need you :) In order to prepare for this task please find out if we will need anything in particular before we can post this amount of letters. The post office need to be capable of accepting letters in bulk and using a franking machine. Perhaps it will be necessary to go to several post offices instead of just one? Are there any post offices open 24h/7 in Madrid? I will arrive on Monday or Tuesday next week (29.05 or 30.05). In order to properly prepare for this job, you will need to find the right Post Office branches, call them or go there personally, ask if there are any limits for sending the letters (letter amount or money limits), also find out if it's possible to pay with CreditCard or cash only, and can i get an invoice for a UK company? Do we need to fill in any forms...? Basically, all formalities must be known before i arrive to ensure the whole process goes smoothly. This is a must before we can continue. Please only apply if you are available throughout Monday and Tuesday.