Low Value Consignment Relief (LVCR) ended this spring after a campaign by retailers on the mainland who did not benefit from the 20 per cent break on mail-order sales of less than £15.
Guernsey Post is expecting revenue from bulk mail to fall by around £16m this year as it counts the cost of losing seven out of its top 10 customers. HMV, CD Magpie, The Hut and 7dayshop.com have moved from the islands and now Thompson & Morgan is "reviewing the costs and benefits" of leaving.
The company leases production, packaging, warehousing and glasshouses on Guernsey. It will decide what to do within a month.
Managing director Paul Hansord said a combination of losing LVCR, bad weather and the poor economy have made trading "very challenging" since T&M increased prices by 20 per cent to cover the VAT benefit loss in April.
He added that "all the horticulture industry that was packing in Guernsey" is looking at pulling out.