All National Garden Gift Voucher (NGGV) orders placed between now and 31 December will be sent using the next day delivery service during the peak Christmas period.
The HTA says members should claim redemptions using a guaranteed courier serviceor if using Royal Mail to avoid posting around strike days.
Many non-store garden retailers now use alternative postal providers to the Royal Mai.
British Retail Council head of non-store retailing Sharon Hardiman said online sales may stop building pre-Christmas months "depending on the postal strikes. Online's slice of overall Christmas spending hinges heavily on customers being sure retailers can get the goods to them. Online shopping is a big source of growth for Royal Mail. We all need common sense to prevail quickly."
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