Redcaps are supposed to spend two seasons/year in service to the Order -- delivering messages and performing similar tasks. This is apparently compatible with a mercantile occupation without any extra loss of time; City and Guild states that the Redcap virtue may substitute for any of the various mercantile social status virtues (Merchant, Merchant Venturer, Factor and Capo) with Redcaps changing roles "at the whim of their House". However, it's not clear at all how this interacts with the Wealth & Labor points rules from City and Guild.
Can a Redcap who has set aside enough labor points to become a Wealthy merchant enjoy all the benefits of the Virtue except he must still work two seasons/year to avoid House censure? I'd say yes.
And what about Lone Redcaps from HoH:TL? According to HoH:TL, these are more independent Redcaps who do not report frequently to the Mercer houses; they do not receive magical items or longevity rituals and have a poor reputation, but being a Lone Redcap is only a Minor Virtue -- and Lone Redcaps who take the Wealthy Virtue are required to spend only a single season in service to the Order.
Can the Lone Redcap Virtue substitute for mercantile social status virtues? I'd say no.
Can one take a mercantile social status virtue (e.g. Merchant Venturer) in addition to Lone Redcap, with the seasons spent as a merchant counting for Redcap service (basically, as the character travels to deliver messages, he can trade)? I'd say yes, since the same can be done by ordinary Redcaps.
If a Lone Redcap merchant earns enough wealth to become Wealthy, can he cut down his work seasons from two to one without suffering censure, as if he had taken the Wealthy virtue at character creation? I'd say no; "Wealthy" for Lone Redcaps means something different than it does for merchants -- more than a measure of affluence it's a measure of how much "free rein" they have within the Order (whether because of past service, personal connections etc.). But I'm conflicted about this.