An online ecommerce shop is just another channel, making it easier for people to order during off-hours such as late and night or on the weekend when your answering service might be active.
However, that's not your core business and as such is an after-thought which will require you to maintain stock level updates, new products being added, old ones removed and generally add a substantial amount of admin overhead into a business workflow which is NOT your core business.
In other words, it's a heap of sand in a world where you want to be focusing on big rocks.
Your service value add is that you're bringing a virtual assistant service to office supply procurement.
As such, your ability to scour multiple sources to find them the best deals is going to save them time, and time is money.
Your knowledge of what brands and products are crap and which ones are the gold standard will save them frustration which they would experience in buying crap and learning hard lessons on what to avoid.
Your finger on the pulse of the market is giving you insights into new products, new features, new deals on old products, so that you can save them money and keep them up to date thanks to your knowledge.
So... Have a look at procurement services and see how they are marketing/selling their services, while also looking at the virtual assistant marketing/sales, since really, you're a combination of the two.
The 5 minute here and there required to find a product, order it, pay for it, adds up. In fact, for companies using PO's, the PO to Pay process averages a labour cost of nearly 100 GBP.
Obviously this isn't true of ordering a box of pens from OfficeMax using the credit card in your pocket, it does take time, especially with multiple products.
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Service value-adds are where you can provide insight into the spend, buying patterns, and so forth in either a regular report, or online dashboard that they can glance at in seconds and see what their cost centres are, where they're potentially needing to look at saving money or change processes, and generally get a better view of their expenses.
A great value add is auto-reordering and/or predictive recommendations. If they've got a regular purchase of x, y or z, and are manually requesting such every week, month, every other month, quarter, etc, then by noticing the pattern you can help them cut out their re-order time.
A friendly reminder email that they might be due for another ream of paper, set of employee badges, binder rings, or whatever, would be welcomed, if they could simply reply and say yes, please reorder.
If you suggest an auto-refill service, they get a new print cartridge at x interval, automatically, until they say cancel. That service would eliminate completely their worrying about inventory levels of office supplies and keep them topped up with zero effort on their part.
More service means smarter service and less effort or expense on their part. You save them time or money or both and they're gonna stick around.