Masking tape is kinda meant to be taken off. Its not something that would really keep a box closed very well. Especially the kind that is like an inch wide.
If it's USPS approved tape, it should say somewhere on the cardboard inside. You can buy a label off Paypal, but you'll need clear tape to tape it down. Or, you can bring it to the post office, where they'll have cheap tape you can use, and they'll print the label for you.
TRUCK- Eating kittens and kicking puppies since 1982.
If you take two boxes and merge them into one, then you can no longer take advantage of the Flat Rate Shipping offer (provided they were flat rate boxes to begin with. If not, any kind of cardboard will do as long as it doesn't have any brand names or markings on it. My suggestion is to seal up the box really well first, and then put a shipping label on it. Then it won't matter what color tape you use.
If your post office has an Automated Postal Center (APC), then simply write the address down and bring it with you to the post office. You can then use the APC that will print out a label with the town, state and zip code (that label also counts as your postage). You can then write the name and street address on the label so long as you can fit it into the blank space above the town. (Very important: make sure that you don't make any marks on the barcode or you will delay shipping and/or risk losing it in the mail). Using the APC has saved me hours of waiting in line at the post office over the past few years of selling stuff on eBay.
Just get a roll of clear packing tape from Wal-Mart or Office Depot. Pack the thing you're shipping in the box with plenty of crumpled-up newspaper and it will be fine. (You might want to protect the screen from scratches, so cut out a couple layers of saran wrap and tape them to the case around the screen.)
(IN B4 BLOGGING)
(And you never specified what exactly it is you're shipping in this thread.)