I'm not a seller on SL but I buy a fair amount of cards from the site. (Actually, if you ask my wife, I buy A LOT of cards from SportLots.) If a seller offers $0.00, or around $1, box shipping I will buy from that seller, especially if I'm not in a hurry to get the cards. I will purchase cards for box shipping and then let them accumulate for almost a year before I have them shipped. I currently have 27 or 28 purchases in my box, the first one was purchased the end of January 2023. I'm waiting for 4 more to arrive before I pay and start again. I think it was $9.50 for shipping after I had 4-5 envelopes and now, with almost 500 cards in the "box" it will be $11. If you're not in a hurry to get the cards, and I'm not usually in a hurry, I think it's a great option.
To answer the original question about how long to wait before shipping: I'd suggest adding a note in the Seller's Shipping Policy section that you will ship box orders every __ weeks. Whatever you decide, the buyer knows in advance what it will be and can make his/her decision from there.