(it's somewhat possible that SAMD21 chips come pre-programmed (from Atmel/Microchip) with a USB/Serial bootloader, so that if all you want to do is load other code, you might not need a programmer. But the included bootloader is probably not quite the Arduino bootloader, and you may need a programmer to replace the bootloader.)
Atmel-ICE is the current "not too expensive" programmer that handles almost all Atmel chips - AVRs with their three+ serial programming protocols, ARMs with JTAG or SWD...