I find spending a bit more on batteries goes along way. Although the nominal voltage and size may be the same, better batteries have lower internal resistance, ie provide the same current with less voltage sag. This prevent the low bat detection from tripping prematurely.
Yeah just say you accept either prepaid with Monero (XMR) or using the cash-on-delivery option. A link to getmonero.org in the FAQ would be appreciated, but is not required.