Do these services gather the mail and save it for a weekly or bi weekly shipment to the DR. Or is every piece sent everyday?
I get the impression that they ship most, if not every day.
If so is the mail held in Miami or in SD?
Santo Domingo.
Is it picked up at the SD office by you or delivered to your home?
Their office.
Can junk mail be tossed?
If you get junk mail and reject it, they keep it and you only pay a small charge, but you are encouraged to contact the sender and get yourself removed from their mailing list. I also registered with the US version of the mailing preference service and have not received any junk mail since. I'm also more careful about checking the opt-out box when I provide my mailing address on a website.
Are costs per piece or a monthly fee?
Annual fee (only about US$10) + cost per piece.
Welcome to POBI (P.O. Box International)
Hope this helps!